JavaScript Basics

Quick Facts

JavaScript is dynamic, untyped, and interpreted

There is no compile step

ALWAYS use semicolons at the ends of statements

comment lines begin with //

• comment blocks start with /* and end with */

console.log() is how you write to the console

You can declare/assign variables in several ways:

1. var x = 10;

2. var x; x=10;

3. var w=10,v=10;

ALWAYS use let or var or const when declaring a variable, otherwise you pollute the global context.

Note:

variables are CaSe SeNsItIvE

Types

Primitive Types/Values

number

• floating point

boolean

• true/false

string

• no char, just strings of length 1
• use .length for string length

null

• used in places where an object can be expected, but no object is relevant
• There’s only one
• Today, it is rationalized that null is considered a placeholder for an object, even though technically it is a primitive value
• NOT an empty object

undefined

• used in places where a variable has been declared, but not assigned
• “error-like” absence of value
• Returned from functions if no value is explicitly returned

symbol

• unique, immutable, primitive value

Object

• A collection of properties
• properties are pairings of key to value
  • key must be a strings
  • values can be anything
• Object literal syntax:
  • x = {id:5, name:"bob", dog:{}}
• Objects can be defined on-the-fly:

x = {};
x.id=5;
x.name = "fred";
x.dog = {};

Assignment

Variables are dynamically typed

You can reassign a var to something of a different type

var x = 12;
console.log(typeof(x)); // number
x = "bob"
console.log(typeof(x)); // string

aside: String is not string

Operators

+, -, *, /, %

• % is remainder, not modulo
• handles sign different

Compound Assignment

+=, -=, *=, /=, %=

Increment/Decrement

++, --

Comparison

>=, <=, <, >, ==, !=

Here be Dragons:

var bar = 5;
bar += 2; // 7
var baz = true;
baz += 1; // true
var foo = true;
foo += false; // 1
1 + "foo"; // "1foo"
"foo" + false; // "foofalse"
"foo" + "bar"; // "foobar"

Arrays

a = ["hey","there",,5];
console.log(a[0]); // "hey"
console.log(a[2]); // undefined

Variable Hoisting

Variable declarations float to the top

Example: (these two are logically equivalent)

console.log(x === undefined); // true
var x = 3;

var x;
console.log(x === undefined); // true
x=3;

When scripts loaded, variable declarations are brought to the top

Scope

Traditionally, blocks don’t have scope, only functions do.

If you declare a variable in an if statement, it is visible to the whole function

let and const allow you to create block-scoped variables