JS Intro.md

JavaScript Intro

JavaScript is a programming language. It was invented in 1995 (in ten days, by one person) as a scripting language to be used in web browsers.

It is the most popular programming language in the world, and now operates in many places besides the browser.

Fact: JavaScript is completely unrelated to Java. Highlight this paragraph and show your speaker notes to the class to illustrate just how important this point is.

Variables

The core operation of any programming language is to execute code based on variables. Consider a box that you can store information in. Variables are information boxes with names.

Variables in JavaScript must be explicitly declared:

var color;

Once declared, you can set (or "define") the variable's value however many times you like:

color = "Blue";
color = "Red";
color = "Green";

Shorthand for doing both at once:

var language = "JavaScript";

Naming

Variable names...

• can only contain letters, numbers, _, and $
• can't start with numbers
• are case-sensitive
• can't be one of the reserved words

When choosing a variable name that's best expressed in English with multiple words, there are two different conventions: joining_words_with_underscores and joiningWordsWithCamelCase.

Neither is wrong. But: choose one and stick with it.

Primitives

There are three basic "types" of values in JavaScript, which are referred to as "primitives."

Strings are collections of characters, expressed inside of quotation marks.

var hello = "Good morning, Kyle!";

Numbers are both integers and "floats" (or decimals).

var positive = 5;
var negative = -20;
var float = 1.421;

Booleans are either true or false.

var ironYardIsAwesome = true;
var rubyIsBetter = false;

Unlike some languages, JavaScript variables are loosely typed, so you can change a variable's type as much as you want.

var x = 42;
x = "Hey, am I a string now?";
x = true;

Comments

// I'm a single line comment
var hello = "world";

/*
  Multi
  line
  comment
*/
var foo = "bar";

Non-Values

A variable has not had a value set for it yet is undefined. Variables can specifically be set to undefined as well.

var empty;

var hadAValue = true;
hadAValue = undefined;

There's another non-value that people use sometimes (null) but don't worry about that for a while.

Resources

• W3C - A History of JavaScript -- the most diplomatic way one can write a history of the first 10ish years of JS
• The Birth and Death of JavaScript - humorous but serious talk about where the language came from and where it's going