awesome-js

console.log(1 +  "2" + "2");
console.log(1 +  +"2" + "2");
console.log(1 +  -"1" + "2");
console.log(+"1" +  "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);

See Answer

1.1.4

What's the difference between using “let” and “var” to declare a variable?

See Answer

1.1.5

What's the difference between null and undefined?

See Answer

1.2 scope

#### 1.2.1

What will the code below output to the console and why?

var myObject = {
    foo: "bar",
    func: function() {
        var self = this;
        console.log("outer func:  this.foo = " + this.foo);
        console.log("outer func:  self.foo = " + self.foo);
        (function() {
            console.log("inner func:  this.foo = " + this.foo);
            console.log("inner func:  self.foo = " + self.foo);
        }());
    }
};
myObject.func();

See Answer

1.2.2

What is the significance of, and reason for, wrapping the entire content of a JavaScript source file in a function block?

See Answer

1.2.3

Consider the following code snippet:

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', function(){ console.log(i); });
  document.body.appendChild(btn);
}

(a) What gets logged to the console when the user clicks on “Button 4” and why?

(b) Provide one or more alternate implementations that will work as expected.

See Answer

1.2.4

What is a “closure” in JavaScript? Provide an example.

See Answer

1.2.5

What will be the output of the following code:

for (var i = 0; i < 5; i++) {
  setTimeout(function() { console.log(i); }, i * 1000 );
}

Explain your answer. How could the use of closures help here?

See Answer

1.2.6

Consider the code snippet below. What will the console output be and why?

(function(x) {
    return (function(y) {
        console.log(x);
    })(2)
})(1);

See Answer

1.2.7

What will the following code output to the console and why:

var hero = {
    _name: 'John Doe',
    getSecretIdentity: function (){
        return this._name;
    }
};

var stoleSecretIdentity = hero.getSecretIdentity;

console.log(stoleSecretIdentity());
console.log(hero.getSecretIdentity());

What is the issue with this code and how can it be fixed.

See Answer

1.2.8

How and why would you use closure?

See Answer

1.3 Error Handling

1.3.1

What is the significance, and what are the benefits, of including 'use strict' at the beginning of a JavaScript source file?

See Answer

1.3.2

Consider the two functions below. Will they both return the same thing? Why or why not?

function foo1()
{
  return {
      bar: "hello"
  };
}

function foo2()
{
  return
  {
      bar: "hello"
  };
}

See Answer

1.3.3

The following recursive code will cause a stack overflow if the array list is too large. How can you fix this and still retain the recursive pattern?

var list = readHugeList();

var nextListItem = function() {
    var item = list.pop();

    if (item) {
        // process the list item...
        nextListItem();
    }
};

See Answer

1.4 Numbers

1.4.1

What will the code below output? Explain your answer.

console.log(0.1 + 0.2);
console.log(0.1 + 0.2 == 0.3);

See Answer

1.4.2

Discuss possible ways to write a function isInteger(x) that determines if x is an integer.

See Answer

1.5 Events and Runtime

1.5.1

In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?

(function() {
    console.log(1); 
    setTimeout(function(){console.log(2)}, 1000); 
    setTimeout(function(){console.log(3)}, 0); 
    console.log(4);
})();

See Answer

1.5.2

What is Javascript's Event Bubbling and Capturing?

See Answer

1.6 Functions

1.6.1

Write a sum method which will work properly when invoked using either syntax below.

console.log(sum(2,3));   // Outputs 5
console.log(sum(2)(3));  // Outputs 5

See Answer

1.6.2

What will the following code output to the console:

console.log((function f(n){return ((n > 1) ? n * f(n-1) : n)})(10));

Explain your answer.

See Answer

1.6.3

Difference between: function Person(){}, var person = Person(), and var person = new Person()?

See Answer

1.6.4

What's the difference between .call and .apply?

See Answer

1.6.5

When would you use document.write()?

See Answer

1.7 Reference

1.7.1

What will the code below output to the console and why?

var arr1 = "john".split('');
var arr2 = arr1.reverse();
var arr3 = "jones".split('');
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));

See Answer

1.8 Comparison and Logical Operators

1.8.1

What would the following lines of code output to the console?

console.log("0 || 1 = "+(0 || 1));
console.log("1 || 2 = "+(1 || 2));
console.log("0 && 1 = "+(0 && 1));
console.log("1 && 2 = "+(1 && 2));

Explain your answer.

See Answer

1.8.2

What will be the output when the following code is executed? Explain.

console.log(false == '0')
console.log(false === '0')

See Answer

1.9 Objects

1.9.1

What is the output out of the following code? Explain your answer.

var a={},
    b={key:'b'},
    c={key:'c'};

a[b]=123;
a[c]=456;

console.log(a[b]);

See Answer

1.9.2

Compare and contrast objects and hashtables in JavaScript.

See Answer

1.9.3

What's the difference between host objects and native objects?

See Answer

1.10 DOM (Document Object Model)

1.10.1

Create a function that, given a DOM Element on the page, will visit the element itself and all of its descendents (not just its immediate children). For each element visited, the function should pass that element to a provided callback function.

The arguments to the function should be:

a DOM element
a callback function (that takes a DOM element as its argument)

See Answer

1.10.2

Given a node from a DOM tree find the node in the same position from an identical DOM tree. See diagram below for clarity.

 A         B

 O        O
 |\       |\
 O O      O O
  /|\      /|\
 O O O    O O O
      \        \
       O        O

See Answer

1.11 Prototype

1.11.1

Describe inheritance and the prototype chain in JavaScript. Give an example.

See Answer

1.11.2

When is prototypal inheritance an appropriate choice?

See Answer

1.11.3

Explain Function.prototype.bind

See Answer

Algorithms

2.1 String Palindrome

Write a simple function (less than 80 characters) that returns a boolean indicating whether or not a string is a palindrome.

See Answer

2.2 Square Root

Manually calculate the square root of a number with Javascript.

See Answer

2.3 Flattening Arrays

Can you write a function that deeply flattens an array?

//example
input = [0, 1, [2, 3], [[4, [5]]]];
output = [0, 1, 2, 3, 4, 5];

See Answer

2.4 Reordering Objects in an Array

We have an array of objects A and an array of indexes B. Reorder objects in array A with given indexes in array B. Do not change array A's length.

//example
var A = [C, D, E, F, G];
var B = [3, 0, 4, 1, 2];

sort(A, B);
// A is now [D, F, G, C, E];

See Answer

2.5 Replacing Substring

Given a string Sting="ABCSC" Check whether it contains a Substring="ABC"?

If no , return "-1".
If yes , remove the substring from string and return "SC".

See Answer

2.6 Move Zeroes

Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements.

For example, given nums = [0, 1, 0, 3, 12], after calling your function, nums should be [1, 3, 12, 0, 0].

Note:

You must do this in-place without making a copy of the array.
Minimize the total number of operations.

See Answer

2.7 Singles

Given an array of integers, every element appears twice except for one. Find that single one.

Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example :

Input : [1 2 2 3 1]
Output : 3

See Answer

##Answers

1.0 Javascript Principles

1.0.1

JavaScript is a multi-paradigm language, supporting imperative/procedural programming along with OOP (Object-Oriented Programming) and functional programming. JavaScript supports OOP with prototypal inheritance.

Javascript is Objects Linking to Other Objects (OLOO).

//imperative
function double (arr) {
  let results = []
  for (let i = 0; i < arr.length; i++){
    results.push(arr[i] * 2)
  }
  return results
}

//functional
function double (arr) {
  return arr.map((item) => item * 2)
}

//functional does not mutate any state, or information held in memory

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1.0.2

Functional programming produces programs by composing mathematical functions and avoids shared state & mutable data. Pure functions don't mutate any state or data.

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1.0.3

**Class Inheritance: **instances inherit from classes (like a blueprint — a description of the class), and create sub-class relationships: hierarchical class taxonomies. Instances are typically instantiated via constructor functions with the new keyword. Class inheritance may or may not use the class keyword from ES6.

Prototypal Inheritance: instances inherit directly from other objects. Instances are typically instantiated via factory functions or Object.create(). Instances may be composed from many different objects, allowing for easy selective inheritance.

composition - your object contains another object, called its prototype, and
delegation - when you access a property on your object, if it's not directly defined on it, javascript will search the prototype chain and delegate that behavior to the first object it finds that defines that property.

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1.0.4

**OOP Pros: **It’s easy to understand the basic concept of objects and easy to interpret the meaning of method calls. OOP tends to use an imperative style rather than a declarative style, which reads like a straight-forward set of instructions for the computer to follow.

OOP Cons: OOP Typically depends on shared state. Objects and behaviors are typically tacked together on the same entity, which may be accessed at random by any number of functions with non-deterministic order, which may lead to undesirable behavior such as race conditions.

FP Pros: Using the functional paradigm, programmers avoid any shared state or side-effects, which eliminates bugs caused by multiple functions competing for the same resources. With features such as the availability of point-free style (aka tacit programming), functions tend to be radically simplified and easily recomposed for more generally reusable code compared to OOP.

FP Cons: Over exploitation of FP features such as point-free style and large compositions can potentially reduce readability because the resulting code is often more abstractly specified, more terse, and less concrete.

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1.0.5

This is a trick question. The answer is never. I’ve been issuing this challenge for years, and the only answers I’ve ever heard fall into one of several common misconceptions. More frequently, the challenge is met with silence.

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1.0.6

This is a quote from “Design Patterns: Elements of Reusable Object-Oriented Software”. It means that code reuse should be achieved by assembling smaller units of functionality into new objects instead of inheriting from classes and creating object taxonomies.

In other words, use can-do, has-a, or uses-a relationships instead of is-a relationships.

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1.0.7

Two way data binding means that UI fields are bound to model data dynamically such that when a UI field changes, the model data changes with it and vice-versa.

One way data flow means that the model is the single source of truth. Changes in the UI trigger messages that signal user intent to the model (or “store” in React). Only the model has the access to change the app’s state. The effect is that data always flows in a single direction, which makes it easier to understand.

One way data flows are deterministic, whereas two-way binding can cause side-effects which are harder to follow and understand.

Good to hear:

React is the new canonical example of one-way data flow, so mentions of React are a good signal. Cycle.js is another popular implementation of uni-directional data flow.
Angular is a popular framework which uses two-way binding.

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1.0.8

A monolithic architecture means that your app is written as one cohesive unit of code whose components are designed to work together, sharing the same memory space and resources.

A microservice architecture means that your app is made up of lots of smaller, independent applications capable of running in their own memory space and scaling independently from each other across potentially many separate machines.

**Monolithic Pros: **The major advantage of the monolithic architecture is that most apps typically have a large number of cross-cutting concerns, such as logging, rate limiting, and security features such audit trails and DOS protection.

When everything is running through the same app, it’s easy to hook up components to those cross-cutting concerns.

There can also be performance advantages, since shared-memory access is faster than inter-process communication (IPC).

Monolithic cons: Monolithic app services tend to get tightly coupled and entangled as the application evolves, making it difficult to isolate services for purposes such as independent scaling or code maintainability.

Monolithic architectures are also much harder to understand, because there may be dependencies, side-effects, and magic which are not obvious when you’re looking at a particular service or controller.

Microservice pros: Microservice architectures are typically better organized, since each microservice has a very specific job, and is not concerned with the jobs of other components. Decoupled services are also easier to recompose and reconfigure to serve the purposes of different apps (for example, serving both the web clients and public API).

They can also have performance advantages depending on how they’re organized because it’s possible to isolate hot services and scale them independent of the rest of the app.

Microservice cons: As you’re building a new microservice architecture, you’re likely to discover lots of cross-cutting concerns that you did not anticipate at design time. A monolithic app could establish shared magic helpers or middleware to handle such cross-cutting concerns without much effort.

In a microservice architecture, you’ll either need to incur the overhead of separate modules for each cross-cutting concern, or encapsulate cross-cutting concerns in another service layer that all traffic gets routed through.

Eventually, even monolthic architectures tend to route traffic through an outer service layer for cross-cutting concerns, but with a monolithic architecture, it’s possible to delay the cost of that work until the project is much more mature.

Microservices are frequently deployed on their own virtual machines or containers, causing a proliferation of VM wrangling work. These tasks are frequently automated with container fleet management tools.

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1.0.9

Synchronous programming means that, barring conditionals and function calls, code is executed sequentially from top-to-bottom, blocking on long-running tasks such as network requests and disk I/O.

Asynchronous programming means that the engine runs in an event loop. When a blocking operation is needed, the request is started, and the code keeps running without blocking for the result. When the response is ready, an interrupt is fired, which causes an event handler to be run, where the control flow continues. In this way, a single program thread can handle many concurrent operations.

User interfaces are asynchronous by nature, and spend most of their time waiting for user input to interrupt the event loop and trigger event handlers.

Node is asynchronous by default, meaning that the server works in much the same way, waiting in a loop for a network request, and accepting more incoming requests while the first one is being handled.

This is important in JavaScript, because it is a very natural fit for user interface code, and very beneficial to performance on the server.

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1.0.10

Hoisting is JavaScript's default behavior of moving declarations to the top. In JavaScript, a variable can be declared after it has been used. In other words; a variable can be used before it has been declared.

Example 1 gives the same result as Example 2:

Example 1:

x = 5; // Assign 5 to x

elem = document.getElementById("demo"); // Find an element 
elem.innerHTML = x;                     // Display x in the element

var x; // Declare x

Example2 :

var x; // Declare x
x = 5; // Assign 5 to x

elem = document.getElementById("demo"); // Find an element 
elem.innerHTML = x;                     // Display x in the element

Don't get confused that ONLY the declaration is moved to the top. Value initialization is done on the same line.

Thus y in this example is undefined:

var x = 5; // Initialize x

elem = document.getElementById("demo"); // Find an element 
elem.innerHTML = x + " " + y;           // Display x and y

var y = 7; // Initialize y

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1.0.11

Over the years there’s been a steadily increasing ecosystem of JavaScript components to choose from. The sheer amount of choices is fantastic, but this also infamously presents a difficulty when components are mixed-and-matched. And it doesn’t take too long for budding developers to find out that not all components are built to play nicely together.

To address these issues, the competing module specs AMD and CommonJS have appeared on the scene, allowing developers to write their code in an agreed-upon sandboxed and modularized way, so as not to “pollute the ecosystem”

Asynchronous Module Definition (AMD) has gained traction on the frontend, with RequireJS being the most popular implementation.

Here’s module foo with a single dependency on jquery:

//    filename: foo.js
define(['jquery'], function ($) {
    //    methods
    function myFunc(){};

    //    exposed public methods
    return myFunc;
});

CommonJS is a style you may be familiar with if you’re written anything in Node (which uses a slight variant). It’s also been gaining traction on the frontend with Browserify.

Using the same format as before, here’s what our foo module looks like in CommonJS:

//    filename: foo.js

//    dependencies
var $ = require('jquery');

//    methods
function myFunc(){};

//    exposed public method (single)
module.exports = myFunc;

Since CommonJS and AMD styles have both been equally popular, it seems there’s yet no consensus. This has brought about the push for a “universal” pattern that supports both styles, which brings us to none other than the Universal Module Definition.

The pattern is admittedly ugly, but is both AMD and CommonJS compatible, as well as supporting the old-style “global” variable definition:

(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
        // AMD
        define(['jquery'], factory);
    } else if (typeof exports === 'object') {
        // Node, CommonJS-like
        module.exports = factory(require('jquery'));
    } else {
        // Browser globals (root is window)
        root.returnExports = factory(root.jQuery);
    }
}(this, function ($) {
    //    methods
    function myFunc(){};

    //    exposed public method
    return myFunc;
}));

More Information

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1.0.12

IIFE is a function that executes immediately after it’s created.

It has nothing to do with any event-handler for any events (such as document.onload). The first pair of parentheses (function(){...}) turns the code within (in this case, a function) into an expression, and the second pair of parentheses (function(){...})() calls the function that results from that evaluated expression.

This pattern is often used when trying to avoid polluting the global namespace, because all the variables used inside the IIFE (like in any other normal function) are not visible outside its scope. This is why, maybe, you confused this construction with an event-handler for window.onload, because it’s often used as this:

(function(){
    // all your code here
    var foo = function() {};
    window.onload = foo;
    // ...
})();
// foo is unreachable here (it’s undefined)

The function is executed right after it's created, not after it is parsed. The entire script block is parsed before any code in it is executed. Also, parsing code doesn't automatically mean that it's executed, if for example the IIFE is inside a function then it won't be executed until the function is called.

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1.0.13

The function is missing the first pair of parantheses that turns the code within into an expression.

//wrong
function foo(){ }();

//correct
(function foo(){})();

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1.0.14

Feature detection checks a feature for existence, e.g.:

if (window.XMLHttpRequest) {
    new XMLHttpRequest();
}

Feature inference checks for a feature just like feature detection, but uses another function because it assumes it will also exist, e.g.:

if (document.getElementsByTagName) {
    element = document.getElementById(id);
}

Checking the UA string is an old practice and should not be used anymore. You keep changing the UA checks and never benefit from newly implemented features, e.g.:

if (navigator.userAgent.indexOf("MSIE 7") > -1){
    //do something
}

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1.0.15

Simply put, AJAX is the use of JavaScript to send and receive using HTTP without reloading the page. AJAX is an acronym for asynchronous JavaScript and XML, and is used as a technique for creating client-side asynchronous web applications. AJAX is considered a group of technologies. HTML and CSS can be used in combination to mark up and style information. JavaScript and the XMLHttpRequest object provide the method for exchanging data asynchronously between the browser and the server.

How does AJAX work?

AJAX, sends and retrieves data from a server asynchronously. This enables the web application to continue running and dynamically display. It allows the user to interact with the information presented on the page, avoiding full page reloads.

The image below shows the process the execution of AJAX.

A user interaction in the browser triggers the event, such as a button click
The AJAX call fires. This creates and AJAX request, browsers use the XMLHttpRequest object. When the server responds to the browser’s request, the same XMLHttpRequest object will process the result.
The server-side script receives the input from JavaScript, and processes the data.
After the data is processed, the script sends the data back to the original client-side page that made the request via XML
Once the data is received, a second JavaScript callback function, is called this function captures the data, and updates the web page accordingly.

Advantages

AJAX provides more efficient and smoother running applications, which gives users better interactive experiences.

The best use of AJAX is where it is used to send small payloads. Here is a simple example.

I load a page that contains information about stock. It has graphs, charts, company information and it also displays the share-price. Every 30 seconds, I make an AJAX request that gets the updated share-price and changes it on the page.

Without AJAX, I might decide to refresh the entire page every 30 seconds, but with AJAX, I can just make a lightweight request to get the tiny bit of information I need.

Disadvantages

Using AJAX to submit forms isn't always the best bet. Asides from not really giving you a clear advantage over posting the form normally, you break conventions such as browser history (although some browsers now include JavaScript "states" as pages in the history).

When using AJAX, you need to handle the task of telling the user if something has gone wrong. You can do this with jQuery by specifying what should happen on error, but many people forget to do this and the end user is blissfully unaware of any problem.

Other issues to watch out for are any JavaScript errors that may prevent your events from firing - or if JavaScript is disabled, in either case ensuring that the form can submit normally before you add the AJAX code is the safest option.

More Information

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1.0.16

Attributes are defined by HTML. Properties are defined by DOM.

Some HTML attributes have 1:1 mapping onto properties. id is one example of such.

Some do not (e.g. the value attribute specifies the initial value of an input, but the valueproperty specifies the current value).

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1.0.17

When you extend an object, you change its behaviour.

Changing the behaviour of an object that will only be used by your own code is fine. But when you change the behaviour of something that is also used by other code there is a risk you will break that other code.

When it comes adding methods to the object and array classes in javascript, the risk of breaking something is very high, due to how javascript works. Long years of experience have taught me that this kind of stuff causes all kinds of terrible bugs in javascript.

If you need custom behaviour, it is far better to define your own class (perhaps a subclass) instead of changing a native one. That way you will not break anything at all.

The ability to change how a class works without subclassing it is an important feature of any good programming language, but it is one that must be used rarely and with caution.

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1.1 typeof

1.1.1

In Javascript, null is also considered an object. Therefore, the code below surprises javascript newcomers.

var bar = null;
console.log(typeof bar === "object"); //logs true

As long as one is aware of this, the problem can easily be avoided by also checking if bar is null:

console.log((bar !== null) && (typeof bar === "object"));  // logs false

To be entirely thorough in our answer, there are two other things worth noting:

First, the above solution will return false if bar is a function. In most cases, this is the desired behavior, but in situations where you want to also return true for functions, you could amend the above solution to be:

console.log((bar !== null) && ((typeof bar === "object") || (typeof bar === "function")));

Second, the above solution will return true if bar is an array (e.g., if var bar = [];). In most cases, this is the desired behavior, since arrays are indeed objects, but in situations where you want to also false for arrays, you could amend the above solution to be:

console.log((bar !== null) && (typeof bar === "object") && (toString.call(bar) !== "[object Array]"));

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1.1.2

The NaN property represents a value that is “not a number”. This special value results from an operation that could not be performed either because one of the operands was non-numeric (e.g., "abc" / 4), or because the result of the operation is non-numeric (e.g., an attempt to divide by zero).

While this seems straightforward enough, there are a couple of somewhat surprising characteristics of NaN that can result in hair-pulling bugs if one is not aware of them.

For one thing, although NaN means “not a number”, its type is, believe it or not, Number:

console.log(typeof NaN === "number");  // logs "true"

Additionally, NaN compared to anything – even itself! – is false:

console.log(NaN === NaN);  // logs "false"

A semi-reliable way to test whether a number is equal to NaN is with the built-in function isNaN(), but even using isNaN() is an imperfect solution.

A better solution would either be to use value !== value, which would only produce true if the value is equal to NaN. Also, ES6 offers a new Number.isNaN() function, which is a different and more reliable than the old global isNaN() function.

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1.1.3

The above code will output the following to the console:

"122"
"32"
"02"
"112"
"NaN2"
NaN

Here’s why…

The fundamental issue here is that JavaScript (ECMAScript) is a loosely typed language and it performs automatic type conversion on values to accommodate the operation being performed. Let’s see how this plays out with each of the above examples.

Example 1: 1 + "2" + "2" Outputs: "122" Explanation: The first operation to be performed in 1 + "2". Since one of the operands ("2") is a string, JavaScript assumes it needs to perform string concatenation and therefore converts the type of 1 to "1", 1 + "2" yields "12". Then, "12" + "2" yields "122".

Example 2: 1 + +"2" + "2" Outputs: "32" Explanation: Based on order of operations, the first operation to be performed is +"2" (the extra + before the first "2" is treated as a unary operator). Thus, JavaScript converts the type of "2" to numeric and then applies the unary + sign to it (i.e., treats it as a positive number). As a result, the next operation is now 1 + 2 which of course yields 3. But then, we have an operation between a number and a string (i.e., 3 and "2"), so once again JavaScript converts the type of the numeric value to a string and performs string concatenation, yielding "32".

Example 3: 1 + -"1" + "2" Outputs: "02" Explanation: The explanation here is identical to the prior example, except the unary operator is - rather than +. So "1" becomes 1, which then becomes -1 when the - is applied, which is then added to 1 yielding 0, which is then converted to a string and concatenated with the final "2" operand, yielding "02".

Example 4: +"1" + "1" + "2" Outputs: "112" Explanation: Although the first "1" operand is typecast to a numeric value based on the unary + operator that precedes it, it is then immediately converted back to a string when it is concatenated with the second "1" operand, which is then concatenated with the final "2" operand, yielding the string "112".

Example 5: "A" - "B" + "2" Outputs: "NaN2" Explanation: Since the - operator can not be applied to strings, and since neither "A" nor "B" can be converted to numeric values, "A" - "B" yields NaN which is then concatenated with the string "2" to yield “NaN2”.

Example 6: "A" - "B" + 2 Outputs: NaN Explanation: As exlained in the previous example, "A" - "B" yields NaN. But any operator applied to NaN with any other numeric operand will still yield NaN.

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1.1.4

The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block (both are global if outside any block), which can be smaller than a function block.

Also, variables declared with let are not accessible before they are declared in their enclosing block.

Global:

They are very similar when used like this outside a function block.

let me = 'go';  // globally scoped
var i = 'able'; // globally scoped

However, global variables defined with let will not be added as properties on the global windowobject like those defined with var.

console.log(window.me); // undefined
console.log(window.i); // 'able'

Function:

They are identical when used like this in a function block.

function ingWithinEstablishedParameters() {
    let terOfRecommendation = 'awesome worker!'; //function block scoped
    var sityCheerleading = 'go!'; //function block scoped
}

Block:

Here is the difference. let is only visible in the for() loop and var is visible to the whole function.

function allyIlliterate() {
    //tuce is *not* visible out here

    for( let tuce = 0; tuce < 5; tuce++ ) {
        //tuce is only visible in here (and in the for() parentheses)
        //and there is a separate tuce variable for each iteration of the loop
    }

    //tuce is *not* visible out here
}

function byE40() {
    //nish *is* visible out here

    for( var nish = 0; nish < 5; nish++ ) {
        //nish is visible to the whole function
    }

    //nish *is* visible out here
}

Redeclaration:

Assuming strict mode, var will let you re-declare the same variable in the same scope. On the other hand, let will not:

'use strict';
let me = 'foo';
let me = 'bar'; // SyntaxError: Identifier 'me' has already been declared
'use strict';
var me = 'foo';
var me = 'bar'; // No problem, `me` is replaced.

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1.1.5

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

var TestVar;
 alert(TestVar); //shows undefined
 alert(typeof TestVar); //shows undefined

null is an assignment value. It can be assigned to a variable as a representation of no value:

 var TestVar = null;
 alert(TestVar); //shows null
 alert(typeof TestVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

 null === undefined // false
 null == undefined // true
 null === null // true

and

 null = 'value' // ReferenceError
 undefined = 'value' // 'value'

From StackOverflow

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1.2 Scope

1.2.1

The above code will output the following to the console:

outer func:  this.foo = bar
outer func:  self.foo = bar
inner func:  this.foo = undefined
inner func:  self.foo = bar

In the outer function, both this and self refer to myObject and therefore both can properly reference and access foo.

In the inner function, though, this no longer refers to myObject. As a result, this.foo is undefined in the inner function, whereas the reference to the local variable self remains in scope and is accessible there.

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1.2.2

This is an increasingly common practice, employed by many popular JavaScript libraries (jQuery, Node.js, etc.). This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries.

Another feature of this technique is to allow for an easily referenceable (presumably shorter) alias for a global variable. This is often used, for example, in jQuery plugins. jQuery allows you to disable the $ reference to the jQuery namespace, using jQuery.noConflict(). If this has been done, your code can still use $ employing this closure technique, as follows:

(function($) { /* jQuery plugin code referencing $ */ } )(jQuery);

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1.2.3

(a) No matter what button the user clicks the number 5 will always be logged to the console. This is because, at the point that the onclick method is invoked (for any of the buttons), the for loop has already completed and the variable i already has a value of 5. (Bonus points for the interviewee if they know enough to talk about how execution contexts, variable objects, activation objects, and the internal “scope” property contribute to the closure behavior.)

(b) The key to making this work is to capture the value of i at each pass through the for loop by passing it into a newly created function object. Here are three possible ways to accomplish this:

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', (function(i) {
    return function() { console.log(i); };
  })(i));
  document.body.appendChild(btn);
}

Alternatively, you could wrap the entire call to btn.addEventListener in the new anonymous function:

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  (function (i) {
    btn.addEventListener('click', function() { console.log(i); });
  })(i);
  document.body.appendChild(btn);
}

Or, we could replace the for loop with a call to the array object’s native forEach method:

['a', 'b', 'c', 'd', 'e'].forEach(function (value, i) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', function() { console.log(i); });
  document.body.appendChild(btn);
});

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1.2.4

A closure is an inner function that has access to the variables in the outer (enclosing) function’s scope chain. The closure has access to variables in three scopes; specifically: (1) variable in its own scope, (2) variables in the enclosing function’s scope, and (3) global variables.

Here is a simple example:

var globalVar = "xyz";

(function outerFunc(outerArg) {
  var outerVar = 'a';
  
  (function innerFunc(innerArg) {
    var innerVar = 'b';
    
    console.log(
      "outerArg = " + outerArg + "\n" +
      "innerArg = " + innerArg + "\n" +
      "outerVar = " + outerVar + "\n" +
      "innerVar = " + innerVar + "\n" +
      "globalVar = " + globalVar);
    
  })(456);
})(123);

In the above example, variables from innerFunc, outerFunc, and the global namespace are all in scope in the innerFunc. The above code will therefore produce the following output:

outerArg = 123
innerArg = 456
outerVar = a
innerVar = b
globalVar = xyz

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1.2.5

The code sample shown will not display the values 0, 1, 2, 3, and 4 as might be expected; rather, it will display 5, 5, 5, 5, and 5.

The reason for this is that each function executed within the loop will be executed after the entire loop has completed and all will therefore reference the last value stored in i, which was 5.

Closures can be used to prevent this problem by creating a unique scope for each iteration, storing each unique value of the variable within its scope, as follows:

for (var i = 0; i < 5; i++) {
	(function(x) {
    	setTimeout(function() { console.log(x); }, x * 1000 );
    })(i);
}

This will produce the presumably desired result of logging 0, 1, 2, 3, and 4 to the console.

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1.2.6

The output will be 1, even though the value of x is never set in the inner function. Here’s why:

As explained in our JavaScript Hiring Guide, a closure is a function, along with all variables or functions that were in-scope at the time that the closure was created. In JavaScript, a closure is implemented as an “inner function”; i.e., a function defined within the body of another function. An important feature of closures is that an inner function still has access to the outer function’s variables.

Therefore, in this example, since x is not defined in the inner function, the scope of the outer function is searched for a defined variable x, which is found to have a value of 1.

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1.2.7

The code will output:

undefined
John Doe

The first console.log prints undefined because we are extracting the method from the hero object, so stoleSecretIdentity() is being invoked in the global context (i.e., the window object) where the _name property does not exist.

One way to fix the stoleSecretIdentity() function is as follows:

var stoleSecretIdentity = hero.getSecretIdentity.bind(hero);

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1.2.8

Passing parameterised behaviour into an algorithm (classic higher-order programming)
Simulating object oriented programming
Implementing exotic flow control, such as jQuery's Event handling and AJAX APIs.

One example of closure that enforces public/private methods:

a = (function () {
    var privatefunction = function () {
        alert('hello');
    }

    return {
        publicfunction : function () {
            privatefunction();
        }
    }
})();

As you can see there, a is now an object, with a method publicfunction ( a.publicfunction() ) which calls privatefunction, which only exists inside the closure. You can NOT call privatefunction directly (i.e. a.privatefunction() ), just publicfunction().

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1.3 Error Handling

1.3.1

the short and most important answer here is that use strict is a way to voluntarily enforce stricter parsing and error handling on your JavaScript code at runtime. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions. In general, it is a good practice.

Some of the key benefits of strict mode include:

Makes debugging easier. Code errors that would otherwise have been ignored or would have failed silently will now generate errors or throw exceptions, alerting you sooner to problems in your code and directing you more quickly to their source.
Prevents accidental globals. Without strict mode, assigning a value to an undeclared variable automatically creates a global variable with that name. This is one of the most common errors in JavaScript. In strict mode, attempting to do so throws an error.
Eliminates this coercion. Without strict mode, a reference to a this value of null or undefined is automatically coerced to the global. This can cause many headfakes and pull-out-your-hair kind of bugs. In strict mode, referencing a a this value of null or undefined throws an error.
Disallows duplicate property names or parameter values. Strict mode throws an error when it detects a duplicate named property in an object (e.g., var object = {foo: "bar", foo: "baz"};) or a duplicate named argument for a function (e.g., function foo(val1, val2, val1){}), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
Makes eval() safer. There are some differences in the way eval() behaves in strict mode and in non-strict mode. Most significantly, in strict mode, variables and functions declared inside of an eval() statement are not created in the containing scope (they are created in the containing scope in non-strict mode, which can also be a common source of problems).
Throws error on invalid usage of delete. The delete operator (used to remove properties from objects) cannot be used on non-configurable properties of the object. Non-strict code will fail silently when an attempt is made to delete a non-configurable property, whereas strict mode will throw an error in such a case.

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1.3.2

Surprisingly, these two functions will not return the same thing. Rather:

console.log("foo1 returns:");
console.log(foo1());
console.log("foo2 returns:");
console.log(foo2());

will yield:

foo1 returns:
Object {bar: "hello"}
foo2 returns:
undefined

Not only is this surprising, but what makes this particularly gnarly is that foo2() returns undefined without any error being thrown.

The reason for this has to do with the fact that semicolons are technically optional in JavaScript (although omitting them is generally really bad form). As a result, when the line containing the return statement (with nothing else on the line) is encountered in foo2(), a semicolon is automatically inserted immediately after the return statement.

No error is thrown since the remainder of the code is perfectly valid, even though it doesn’t ever get invoked or do anything (it is simply an unused code block that defines a property bar which is equal to the string "hello").

This behavior also argues for following the convention of placing an opening curly brace at the end of a line in JavaScript, rather than on the beginning of a new line. As shown here, this becomes more than just a stylistic preference in JavaScript.

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1.3.3

The potential stack overflow can be avoided by modifying the nextListItem function as follows:

var list = readHugeList();

var nextListItem = function() {
    var item = list.pop();

    if (item) {
        // process the list item...
        setTimeout( nextListItem, 0);
    }
};

The stack overflow is eliminated because the event loop handles the recursion, not the call stack. When nextListItem runs, if item is not null, the timeout function (nextListItem) is pushed to the event queue and the function exits, thereby leaving the call stack clear. When the event queue runs its timed-out event, the next item is processed and a timer is set to again invoke nextListItem. Accordingly, the method is processed from start to finish without a direct recursive call, so the call stack remains clear, regardless of the number of iterations.

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1.4 Numbers

1.4.1

An educated answer to this question would simply be: “You can’t be sure. it might print out “0.3” and “true”, or it might not. Numbers in JavaScript are all treated with floating point precision, and as such, may not always yield the expected results.”

The example provided above is classic case that demonstrates this issue. Surprisingly, it will print out:

0.30000000000000004
false

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1.4.2

This may sound trivial and, in fact, it is trivial with ECMAscript 6 which introduces a new Number.isInteger() function for precisely this purpose. However, prior to ECMAScript 6, this is a bit more complicated, since no equivalent of the Number.isInteger() method is provided.

The issue is that, in the ECMAScript specification, integers only exist conceptually; i.e., numeric values are always stored as floating point values.

With that in mind, the simplest and cleanest pre-ECMAScript-6 solution (which is also sufficiently robust to return false even if a non-numeric value such as a string or null is passed to the function) would be the following:

function isInteger(x) { return (x^0) === x; }

The following solution would also work, although not as elegant as the one above:

function isInteger(x) { return Math.round(x) === x; }

Note that Math.ceil() or Math.floor() could be used equally well (instead of Math.round()) in the above implementation.

Or alternatively:

function isInteger(x) { return (typeof x === 'number') && (x % 1 === 0); }

One fairly common incorrect solution is the following:

function isInteger(x) { return parseInt(x, 10) === x; }

While this parseInt-based approach will work well for many values of x, once x becomes quite large, it will fail to work properly. The problem is that parseInt() coerces its first parameter to a string before parsing digits. Therefore, once the number becomes sufficiently large, its string representation will be presented in exponential form (e.g., 1e+21). Accordingly, parseInt() will then try to parse 1e+21, but will stop parsing when it reaches the e character and will therefore return a value of 1. Observe:

> String(1000000000000000000000)
'1e+21'

> parseInt(1000000000000000000000, 10)
1

> parseInt(1000000000000000000000, 10) === 1000000000000000000000
false

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1.5 Events and Runtime

1.5.1

The values will be logged in the following order:

Let’s first explain the parts of this that are presumably more obvious:

1 and 4 are displayed first since they are logged by simple calls to console.log() without any delay
2 is displayed after 3 because 2 is being logged after a delay of 1000 msecs (i.e., 1 second) whereas 3 is being logged after a delay of 0 msecs.

OK, fine. But if 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away? And, if so, shouldn’t it be logged before 4, since 4 is being logged by a later line of code?

The answer has to do with properly understanding JavaScript events and timing.

The browser has an event loop which checks the event queue and processes pending events. For example, if an event happens in the background (e.g., a script onload event) while the browser is busy (e.g., processing an onclick), the event gets appended to the queue. When the onclick handler is complete, the queue is checked and the event is then handled (e.g., the onload script is executed).

Similarly, setTimeout() also puts execution of its referenced function into the event queue if the browser is busy.

When a value of zero is passed as the second argument to setTimeout(), it attempts to execute the specified function “as soon as possible”. Specifically, execution of the function is placed on the event queue to occur on the next timer tick. Note, though, that this is not immediate; the function is not executed until the next tick. That’s why in the above example, the call to console.log(4) occurs before the call to console.log(3) (since the call to console.log(3) is invoked via setTimeout, so it is slightly delayed).

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1.5.2

DOM elements can be nested inside each other. And somehow, the handler of the parent works even if you click on it’s child.

The reason is event bubbling.

For example, the following DIV handler runs even if you click a nested tag like EM or CODE:

<div onclick="alert('Div handler worked!')">
  <em>Click here triggers on nested <code>EM</code>, not on <code>DIV</code></em>
</div>

Bubbling

The main principle of bubbling states: After an event triggers on the deepest possible element, it then triggers on parents in nesting order.

Stopping the bubbling

The bubbling goes right to the top. When an event occurs on an element - it will bubble up to <HTML>, triggering handlers on it’s way.

But a handler may decide that event is fully processed and stop the bubbling.

For W3C-compliant browsers: event.stopPropagation()
For IE<9: event.cancelBubble = true

Capturing

In all browsers, except IE<9, there are two stages of event processing.

The event first goes down - that’s called capturing, and then bubbles up. This behavior is standartized in W3C specification.

Summary

Events first are captured down to deepest target, then bubble up. In IE<9 they only bubble.
All handlers work on bubbling stage excepts addEventListener with last argument true, which is the only way to catch the event on capturing stage.
Bubbling/capturing can be stopped by event.cancelBubble=true (IE) or event.stopPropagation() for other browsers.

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1.6 Functions

1.6.1

There are (at least) two ways to do this:

METHOD 1

function sum(x) {
  if (arguments.length == 2) {
    return arguments[0] + arguments[1];
  } else {
    return function(y) { return x + y; };
  }
}

In JavaScript, functions provide access to an arguments object which provides access to the actual arguments passed to a function. This enables us to use the length property to determine at runtime the number of arguments passed to the function.

If two arguments are passed, we simply add them together and return.

Otherwise, we assume it was called in the form sum(2)(3), so we return an anonymous function that adds together the argument passed to sum() (in this case 2) and the argument passed to the anonymous function (in this case 3).

METHOD 2

function sum(x, y) {
  if (y !== undefined) {
    return x + y;
  } else {
    return function(y) { return x + y; };
  }
}

When a function is invoked, JavaScript does not require the number of arguments to match the number of arguments in the function definition. If the number of arguments passed exceeds the number of arguments in the function definition, the excess arguments will simply be ignored. On the other hand, if the number of arguments passed is less than the number of arguments in the function definition, the missing arguments will have a value of undefined when referenced within the function. So, in the above example, by simply checking if the 2nd argument is undefined, we can determine which way the function was invoked and proceed accordingly.

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1.6.2

The code will output the value of 10 factorial (i.e., 10!, or 3,628,800).

Here’s why:

The named function f() calls itself recursively, until it gets down to calling f(1) which simply returns 1. Here, therefore, is what this does:

f(1): returns n, which is 1
f(2): returns 2 * f(1), which is 2
f(3): returns 3 * f(2), which is 6
f(4): returns 4 * f(3), which is 24
f(5): returns 5 * f(4), which is 120
f(6): returns 6 * f(5), which is 720
f(7): returns 7 * f(6), which is 5040
f(8): returns 8 * f(7), which is 40320
f(9): returns 9 * f(8), which is 362880
f(10): returns 10 * f(9), which is 3628800

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1.6.3

**function Person() {} **

Declares a function (but does not execute it). It will usually have some code between the curly brackets.

var person = Person()

Declares a variable (person), invokes a function (Person) and sets the value of person to the return of the function.

var person = new Person()

Creates a new instance of an object based on the Person function. So the variable (person) is now an Object, not just a string or a number.

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1.6.4

The difference is that apply lets you invoke the function with arguments as an array; callrequires the parameters be listed explicitly. A useful mnemonic is "A for array and C for comma."

See MDN's documentation on apply and call.

Pseudo syntax:

theFunction.apply(valueForThis, arrayOfArgs)

theFunction.call(valueForThis, arg1, arg2, ...)

There is also, as of ES6, the possibility to spread the array for use with the call function, you can see the compatibilities here.

Sample code:

function theFunction(name, profession) {
    console.log("My name is " + name + " and I am a " + profession + ".");
}
theFunction("John", "fireman");
theFunction.apply(undefined, ["Susan", "school teacher"]);
theFunction.call(undefined, "Claude", "mathematician");
theFunction.call(undefined, ...["Matthew", "physicist"]); // used with the spread operator

// Output: 

// My name is John and I am a fireman.
// My name is Susan and I am a school teacher.
// My name is Claude and I am a mathematician.
// My name is Matthew and I am a physicist.

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1.6.5

A traditional script tag will block the page while it is loading and executing. A script loaded with document.write will work asynchronously. That's why you see this on ads or analytics, as such scripts don't influence the page content directly.

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1.7 Reference

1.7.1

The logged output will be:

"array 1: length=5 last=j,o,n,e,s"
"array 2: length=5 last=j,o,n,e,s"

arr1 and arr2 are the same after the above code is executed for the following reasons:

Calling an array object’s reverse() method doesn’t only return the array in reverse order, it also reverses the order of the array itself (i.e., in this case, arr1).
The reverse() method returns a reference to the array itself (i.e., in this case, arr1). As a result, arr2 is simply a reference to (rather than a copy of) arr1. Therefore, when anything is done to arr2 (i.e., when we invoke arr2.push(arr3);), arr1 will be affected as well since arr1 and arr2 are simply references to the same object.

And a couple of side points here that can sometimes trip someone up in answering this question:

Passing an array to the push() method of another array pushes that entire array as a single element onto the end of the array. As a result, the statement arr2.push(arr3); adds arr3 in its entirety as a single element to the end of arr2 (i.e., it does not concatenate the two arrays, that’s what the concat() method is for).
Like Python, JavaScript honors negative subscripts in calls to array methods like slice() as a way of referencing elements at the end of the array; e.g., a subscript of -1 indicates the last element in the array, and so on.

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1.8 Comparison and Logical Operators

1.8.1

The code will output the following four lines:

0 || 1 = 1
1 || 2 = 1
0 && 1 = 0
1 && 2 = 2

In JavaScript, both || and && are logical operators that return the first fully-determined “logical value” when evaluated from left to right.

The or (||) operator. In an expression of the form X||Y, X is first evaluated and interpreted as a boolean value. If this boolean value is true, then true (1) is returned and Y is not evaluated, since the “or” condition has already been satisfied. If this boolean value is “false”, though, we still don’t know if X||Y is true or false until we evaluate Y, and interpret it as a boolean value as well.

Accordingly, 0 || 1 evaluates to true (1), as does 1 || 2.

The and (&&) operator. In an expression of the form X&&Y, X is first evaluated and interpreted as a boolean value. If this boolean value is false, then false (0) is returned and Y is not evaluated, since the “and” condition has already failed. If this boolean value is “true”, though, we still don’t know if X&&Y is true or false until we evaluate Y, and interpret it as a boolean value as well.

However, the interesting thing with the && operator is that when an expression is evaluated as “true”, then the expression itself is returned. This is fine, since it counts as “true” in logical expressions, but also can be used to return that value when you care to do so. This explains why, somewhat surprisingly, 1 && 2 returns 2 (whereas you might it expect it to return true or 1).

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1.8.2

The code will output:

true
false

In JavaScript, there are two sets of equality operators. The triple-equal operator === behaves like any traditional equality operator would: evaluates to true if the two expressions on either of its sides have the same type and the same value. The double-equal operator, however, tries to coerce the values before comparing them. It is therefore generally good practice to use the === rather than ==. The same holds true for !== vs !=.

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1.9 Objects

1.9.1

The output of this code will be 456 (not 123).

The reason for this is as follows: When setting an object property, JavaScript will implicitly stringify the parameter value. In this case, since b and c are both objects, they will both be converted to "[object Object]". As a result, a[b] anda[c] are both equivalent to a["[object Object]"] and can be used interchangeably. Therefore, setting or referencing a[c] is precisely the same as setting or referencing a[b].

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1.9.2

This is somewhat of a trick question since, in JavaScript, objects essentially are hashtables; i.e., collections of name-value pairs. In these name-value pairs, a crucial point to be aware of is that the names (a.k.a., keys) are always strings.

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1.9.3

Both terms are defined in the ECMAScript specfication:

native object

object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment.

NOTE Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.

Source: http://es5.github.com/#x4.3.6

host object

object supplied by the host environment to complete the execution environment of ECMAScript.

NOTE Any object that is not native is a host object.

Source: http://es5.github.com/#x4.3.8

uilt-in objects: String, Math, RegExp, Object, Function etc. - core predefined objects always available in JavaScript. Defined in the ECMAScript spec.

Host objects: objects like window, XmlHttpRequest, DOM nodes and so on, which is provided by the browser environment. They are distinct from the built-in objects because not all environment will have the same host objects. If JavaScript runs outside of the browser, for example as server side scripting language like in Node.js, different host objects will be available.

User objects: objects defined in JavaScript code. So 'Bird' in your example would be a user object.

The JavaScript spec groups built-in objects and user objects together as native objects. This is an unorthodox use of the term "native", since user objects are obviously implemented in JavaScript while the built-ins is most likely implemented in a different language under the hood, just as the host objects would be. But from the perspective of the JavaScript spec, both builtins and user objects are native to JavaScript because they are defined in the JavaScript spec, while host objects are not.

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1.10 DOM (Document Object Model)

1.10.1

Visiting all elements in a tree (DOM) is a classic Depth-First-Search algorithm application. Here’s an example solution:

function Traverse(p_element,p_callback) {
   p_callback(p_element);
   var list = p_element.children;
   for (var i = 0; i < list.length; i++) {
       Traverse(list[i],p_callback);  // recursive call
   }
}

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1.10.2

Given two trees which have identical structure, and a specified node within the first tree, locate the node in the second tree with the same position within the structure.

If we have no other information about the two trees then the position of each node can be characterized as a path from the root node where each step in the path is specified as an index into the childNode array.

function indexOf(arrLike, target) {
    return Array.prototype.indexOf.call(arrLike, target);
}

// Given a node and a tree, extract the nodes path 
function getPath(root, target) {
    var current = target;
    var path = [];
    while(current !== root) {
        path.unshift(indexOf(current.parentNode.childNodes, current));
        current = current.parentNode;
    }
    return path;
}

// Given a tree and a path, let's locate a node
function locateNodeFromPath(root, path) {
    var current = root;
    for(var i = 0, len = path.length; i < len; i++) {
        current = current.childNodes[path[i]];
    }
    return current;
}

function getDoppleganger(rootA, rootB, target) {
    return locateNodeFromPath(rootB, getPath(rootA, target));
}

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1.11 Prototype

1.11.1

Although JavaScript is an object-oriented language, it is prototype-based and does not implement a traditional class-based inheritance system.

All JavaScript objects inherit the properties and methods from their prototype.

In JavaScript, each object internally references another object, called its prototype. That prototype object, in turn, has a reference to its prototype object, and so on. At the end of this prototype chain is an object with null as its prototype. The prototype chain is the mechanism by which inheritance – prototypal inheritance to be precise – is achieved in JavaScript. In particular, when a reference is made to a property that an object does not itself contain, the prototype chain is traversed until the referenced property is found (or until the end of the chain is reached, in which case the property is undefined).

Here’s a simple example:

function Animal() { this.eatsVeggies = true; this.eatsMeat = false; }

function Herbivore() {}
Herbivore.prototype = new Animal();

function Carnivore() { this.eatsMeat = true; }
Carnivore.prototype = new Animal();

var rabbit = new Herbivore();
var bear = new Carnivore();

console.log(rabbit.eatsMeat);   // logs "false"
console.log(bear.eatsMeat);     // logs "true"

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1.11.2

There is more than one type of prototypal inheritance:

Delegation (i.e., the prototype chain).
Concatenative (i.e. mixins, Object.assign()).
- Concatenative inheritance is the process of copying the properties from one object to another, without retaining a reference between the two objects. It relies on JavaScript’s dynamic object extension feature.
  
  Cloning is a great way to store default state for objects: This process is commonly achieved using Object.assign(). Prior to ES6, it was common to use similar .extend() methods from Lodash, Underscore, or jQuery.
Functional (Not to be confused with functional programming. A function used to create a closure for private state/encapsulation).

Each type of prototypal inheritance has its own set of use-cases, but all of them are equally useful in their ability to enable composition, which creates has-a or uses-a or can-do relationships as opposed to the is-a relationship created with class inheritance.

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1.11.3

Bind creates a new function that will have this set to the first parameter passed to bind().

Here's an example that shows how to use bind to pass a member method around that has the correct this:

var Button = function(content) { 
  this.content = content;
};
Button.prototype.click = function() {
  console.log(this.content + ' clicked');
}

var myButton = new Button('OK');
myButton.click();

var looseClick = myButton.click;
looseClick(); // not bound, 'this' is not myButton - it is the global object

var boundClick = myButton.click.bind(myButton);
boundClick(); // bound, 'this' is myButton

Which prints out:

OK clicked
undefined clicked
OK clicked

You can also add extra parameters after the 1st (this) parameter and bind will pass in those values to the original function. Any additional parameters you later pass to the bound function will be passed in after the bound parameters:

// Example showing binding some parameters
var sum = function(a, b) {
  return a + b;
};

var add5 = sum.bind(null, 5);
console.log(add5(10));

Which prints out:

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2.1 String Palindrome

The following one line function will return true if str is a palindrome; otherwise, it returns false.

function isPalindrome(str) {
    str = str.replace(/\W/g, '').toLowerCase(); //removes all non-word character
    return (str == str.split('').reverse().join('')); //remove space, reverse, and check
}

For example:

console.log(isPalindrome("level"));                   // logs 'true'
console.log(isPalindrome("levels"));                  // logs 'false'
console.log(isPalindrome("A car, a man, a maraca"));  // logs 'true'

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2.2 Square Root

To integer value

function sqrt(number) {
    for (let i = 0; i * i <= number; i++) {
        if (i * i === number)
            return i;
   }
   return number; // don't know if you should have this line in case nothing found
}

To precise decimal point

//@param number number to find the square root of
//@param guess number of guesses
function sqrt(number, guess) {
    if (!guess) {
        // Take an initial guess at the square root
        guess = number / 2.0;
    }
    var d = number / guess;              // Divide our guess into the number
    var new_guess = (d + guess) / 2.0;     // Use average of guess and d as our new guess
    if (guess == new_guess) {          
        // The new guess is the same as the old guess; further guesses
        // can get no more accurate so we return this guess
        return guess;
    }
    // Recursively solve for closer and closer approximations of the square root
    return sqrt(number, new_guess);
}

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2.3 Flattening Arrays

function flatten(input) {
    let out = [];
    let loop = function(arr) {
        return arr.map(function(v) {
            return Array.isArray(v)? loop(v) : out.push(v);
        });
    }
    loop(input);
    return out;
}

OR

var list1 = [[0, 1], [2, 3], [4, 5]];
var list2 = [0, [1, [2, [3, [4, [5]]]]]];

const flatten = arr => arr.reduce((a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []);
flatten(list1); // returns [0, 1, 2, 3, 4, 5]
flatten(list2); // returns [0, 1, 2, 3, 4, 5]

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2.4 Reordering Objects in an Array

function sort(A, B) {
	let result = [];
	B.map((index, i) => {
		result[index] = A[i];
	})
	return result;
}

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2.5 Replacing Substring

var string = "ABCSC"; 

return ((string.indexOf("ABC") != -1) ? (string.replace(/ABC/gm,"")) : (-1));

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2.6 Move Zeroes

/**
 * @param {number[]} nums
 * @return {void} Do not return anything, modify nums in-place instead.
 */
var moveZeroes = function(nums) {
    let j = 0;
    nums.map((num, i) => {
        if(num !== 0) { //if number is not 0
          	//swap the num[i] and num[j]
            let temp = nums[j];
            nums[j] = nums[i];
            nums[i] = temp;
            j++;
        }
    })
};

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2.7 Singles

^= is a XOR bitwise operator. Find more information about XOR here

module.exports = { 
	//param A : array of integers
	//return an integer
	singleNumber : function(A){
	    var a;
	    A.forEach(function(item){
	      a ^= item;  
	    });
	    return a;
	}
};

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awesome-js

1.0 Javascript Principles

1.0.1

1.0.2

1.0.3

1.0.4

1.0.5

1.0.6

1.0.7

1.0.8

1.0.9

1.0.10

1.0.11

1.0.12

1.0.13

1.0.14

1.0.15

1.0.16

1.0.17

1.1 typeof

1.1.1

1.1.2

1.1.3

1.1.4

1.1.5

1.2 scope

1.2.2

1.2.3

1.2.4

1.2.5

1.2.6

1.2.7

1.2.8

1.3 Error Handling

1.3.1

1.3.2

1.3.3

1.4 Numbers

1.4.1

1.4.2

1.5 Events and Runtime

1.5.1

1.5.2

1.6 Functions

1.6.1

1.6.2

1.6.3

1.6.4

1.6.5

1.7 Reference

1.7.1

1.8 Comparison and Logical Operators

1.8.1

1.8.2

1.9 Objects

1.9.1

1.9.2

1.9.3

1.10 DOM (Document Object Model)

1.10.1

1.10.2

1.11 Prototype

1.11.1

1.11.2

1.11.3

Algorithms

2.1 String Palindrome

2.2 Square Root

2.3 Flattening Arrays

2.4 Reordering Objects in an Array

2.5 Replacing Substring