JavaScript.md

JavaScript

Useful Methods

Method	Definition	Link
parseInt(Value, radix)	Accept the string and convert it into an integer	geeksforgeeks
num.toString(base)	Used with a number and converts the number to a string	geeksforgeeks
Math.random()	Return a floating-point pseudo-random number between range [0,1) , 0 (inclusive) and 1 (exclusive)	geeksforgeeks
array1.concat(array2, array3, ..., arrayX)	Used to join two or more arrays	w3s
array.fill("value")	Fills the specified elements in an array with a static value	w3s
var a = fruits.indexOf("Apple");	Searches the array for the specified item, and returns its position. Returns -1 if the item is not found.If the item is present more than once, the indexOf method returns the position of the first occurrence.	w3s
array.pop()'	Removes the last element of an array, and returns that element	w3s
array.push("value");	Adds new items to the end of an array, and returns the new length. The new item(s) will be added at the end of the array.	w3s
array.reverse();	Reverses the order of the elements in an array	w3s
array.shift()	Removes the first item of an array. this method changes the length of the array. The return value of the shift method is the removed item	w3s
var a = array.slice(1, 3);	Returns the selected elements in an array, as a new array object. Selects the elements starting at the given start argument, and ends at, but does not include, the given end argument.	w3s
array.sort();	Sorts the items of an array (alphabetic/numeric, ascending/descending)	w3s
array.splice( index, remove_count, item_list )	Modify the contents of an array by removing the existing elements and/or by adding new elements.	geeksforgeeks
array.unshift("item1", "item2")	Adds new items to the beginning of an array, returns the new length	w3s
var d = new Date(); var n = d.getDate();	Returns the day of the month (from 1 to 31) for the specified date	w3s
var n = Date.now();	Returns the number of milliseconds since January 1, 1970 00:00:00 UTC	w3s
Math.abs(-299)	Returns the absolute value of a number	w3s
Math.ceil(1.4) > 2	Rounds a number upwards to the nearest integer, and returns the result	w3s
Math.floor(1.6) > 1	Rounds a number downwards to the nearest integer, and returns the result	w3s
Math.max(val1, val2);	Returns the number with the highest value	w3s
Math.min(val1, val2);	Returns the number with the lowest value	w3s
Math.pow(int, exponent)	Returns the value of x to the power of y	w3s
Math.round(val);	Rounds a number to the nearest integer	w3s
Math.trunc(val);	Returns the integer part of a number	w3s
Number.isFinite(123) //true	Determines whether a value is a finite number	w3s
Number.isInteger(Infinity) //false	Determines whether a value is an integer	w3s
Number.isNaN(0) //false	Determines whether a value is NaN (Not-A-Number)	w3s
var num = 5.56789; var n = num.toFixed(2);	Converts a number into a string, keeping a specified number of decimals	w3s
var str = "We are free"; var patt = new RegExp("e"); var res = patt.test(str);	Tests for a match in a string (in this case, search the string for 'e')	w3s Regex
var str = "HELLO"; var res = str.charAt(0);	Returns the character at the specified index in a string	w3s
var str1 = "Hello "; var str2 = "world!"; var res = str1.concat(str2);	Join two or more strings	w3s
var str = "Welcome to the universe."; var n = str.endsWith("universe.");	Determines whether a string ends with the characters of a specified string	w3s
var str = "Welcome to the universe."; var n = str.includes("world");	Determines whether a string contains the characters of a specified string	w3s
var str = "Hello world!"; str.repeat(2);	Returns a new string with a specified number of copies of the string it was called on	w3s
var str = "Visit Microsoft!"; var res = str.replace("Microsoft", "W3Schools");	Searches a string for a specified value, or a regular expression, and returns a new string where the specified values are replaced	w3s
var str = "Visit W3Schools!"; var n = str.search("W3Schools");	Searches a string for a specified value, and returns the position of the match	w3s
var str = "Hello world!"; var res = str.slice(0, 5);	Extracts parts of a string and returns the extracted parts in a new string. Use the start and end parameters to specify the part of the string you want to extract.	w3s
var str = "How are you doing today?"; var res = str.split(" ");	Split a string into an array of substrings, and returns the new array. note: If an empty string ("") is used as the separator, the string is split between each character.	w3s
var str = "Welcome to the universe."; var n = str.startsWith("Welcome");	Determines whether a string begins with the characters of a specified string	w3s
var str = "Hello world!"; var res = str.substr(1, 4);	Extracts parts of a string, beginning at the character at the specified position, and returns the specified number of characters	w3s
var str = "Hello world!"; var res = str.substring(1, 4);	Extracts the characters from a string, between two specified indices, and returns the new sub string	w3s
var str = "Hello World!"; var res = str.toLowerCase();	Converts a string to lowercase letters	w3s
var str = "Hello World!"; var res = str.toUpperCase();	Converts a string to uppercase letters	w3s
var str = " Hello World! "; alert(str.trim());	Removes whitespace from both sides of a string	w3s

Functions

Function Template

function functionname() {
    // code
}

functionname();

For Loop Template

var numberarray = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (var i = 0; i < numberarray.length; i++) {
    // loop 10 times
}

Fibonacci Sequence

function fibonacci(num){
    var a = 1, b = 0, temp;

    while (num >= 0){
        temp = a;
        a = a + b;
        b = temp;
        num--;
    }

return b;
}

Fibonacci Sequence Recursion

function fibonacci(num) {
    if (num <= 1){
        return 1;
    }

return fibonacci(num - 1) + fibonacci(num - 2);
}

Switch Example

switch(expression) {
    case n:
        code block
        break;
    case n:
        code block
        break;
    default:
        default code block
}

Dictionaries

Create dictionary

var dict = {};

Populate Dictionary

dict["key"] = value;
// or
dict.key = "value";

Iterating Key/Value Pairs

for (var key in dict) {
    var value = dict[key];
}

Dictionary Example

var numberOfOccurences = function(array) {
    var dict = {};
    // check number of occurrences in array
    for(var i = 0; i < array.length; i++) {
        if (array[i] in dict){
        // if the key (array value) is in dict, add
            dict[array[i]]++;
        } else {
        // if the key isn't already in dict, give it the value 1
            dict[array[i]] = 1;
        }
    }
    // display dict with key:value pairs
    console.log(dict);
    for(var key in dict){
        var value = dict[key];
        // console.log the values only
        console.log(value);
    }
};

Terms

Term	Definition	Link
Prototype	Allows you to add new properties/methods to object constructors. Every object in JavaScript has a hidden internal property called Prototype that can be used to extend object properties and methods.	https://www.w3schools.com/js/js_object_prototypes.asp
Inheritance	inheritance is the process by which objects of one class acquire the properties of objects of another class. It supports the concept of hierarchical classification. Inheritance provides re usability. This means that we can add additional features to an existing class without modifying it.	https://www.freecodecamp.org/news/a-guide-to-prototype-based-class-inheritance-in-javascript-84953db26df0/
Classes	A JavaScript class is a type of function. Classes are declared with the class keyword.
Closure	JavaScript variables can belong to the local or global scope. Global variables can be made local (private) with closures. A closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.	https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-closure-b2f0d2152b36
Object	In JavaScript, almost "everything" is an object. In real life, a car is an object. A car has properties like weight and color, and methods like start and stop. Objects are variables too. But objects can contain many values. The values are written as name:value pairs (name and value separated by a colon). Objects can also have methods. Methods are actions that can be performed on objects. Methods are stored in properties as function definitions. In a function definition, this refers to the "owner" of the function.	https://www.w3schools.com/js/js_objects.asp https://www.w3schools.com/js/js_object_definition.asp
Arity	By calling return functionName.length; it will return the number of arguments, which is the functions arity.	https://stackoverflow.com/questions/4848149/get-a-functions-arity
Hoisting	Hoisting is JavaScript's default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function).	https://www.w3schools.com/js/js_hoisting.asp
Scoping	Variables declared within a JavaScript function, become LOCAL to the function. Local variables have Function scope: They can only be accessed from within the function.	https://www.w3schools.com/js/js_scope.asp
Callback	We can pass a function as an argument in another function and later execute that passed-in function or even return it to be executed later.
Polymorphism	Object-oriented paradigm that provides a way to perform a single action in different forms. It provides an ability to call the same method on different JavaScript objects. As JavaScript is not a type-safe language, we can pass any type of data members with the methods.	https://www.javatpoint.com/javascript-oops-polymorphism
Encapsulation	Enclosing all the functionalities of an object within that object so that the object’s internal workings (its methods and properties) are hidden from the rest of the application. This allows us to abstract or localize specific set of functionalities on objects.
Instance	An implementation of a Function.

References

JS Glossary
JS Vocab
JS Jargon