JavaScript

Basics

A Statement is a group of words, numbers, and operators that perform a specific task.
An expression is any reference to a variable or value, or a set of variable(s) and value(s) combined with operators.
Statements are made up of one or more expressions. a = b * 2; This statement has four expressions in it
It’s typically asserted that JavaScript is interpreted, however, the JavaScript engine actually compiles the program on the fly and then immediately runs the compiled code.

Comments

Comments should explain why, not what. They can optionally explain how if what’s written is particularly confusing.

// Single line comment

/*
* Multiline
* Comment
*/

Declarations

var Declares a variable, optionally initializing it to a value.

Weak typing, otherwise known as dynamic typing, allows a variable to hold any type of value at any time.

By convention, JavaScript variables as constants are usually capitalized, with underscores _ between multiple words.

let Declares a block scope local variable, optionally initializing it to a value.
const Declares a read-only named constant.

Blocks

A Block is a way to group a series of statements together.
In JavaScript, a block is defined by wrapping one or more statements inside a curly-brace pair { .. }.

Operators

Operators are how we perform actions on variables and values.

Assignment operators

Name	Shorthand operator	Meaning
Assignment	x = y	x = y
Addition assignment	x += y	x = x + y
Subtraction assignment	x -= y	x = x - y
Multiplication assignment	x *= y	x = x * y
Division assignment	x /= y	x = x / y
Remainder assignment	x %= y	x = x % y
Exponentiation assignment	x **= y	x = x ** y

Comparison operators
Arithmetic operators
Bitwise operators
Logical operators
String operators
Conditional (ternary) operator
Comma operator
Unary operators
Relational operator

Data Types

A primitive is data that is not an object and has no methods.
All primitives are immutable.

Type	typeof evaluation	Primitive
Boolean	boolean	yes
Number	number	yes
String	string	yes
Symbol	symbol	yes
null	object	yes
undefined	undefined	yes
Object	object	no
Function	function	no
Array	object	no

Type Coercion

Coercion is the ability to convert between types. Coercion can be Implicit or Explicit

Falsy Values

""
0
null
undefined
NaN
false

Variables

Naming Convention

Reserved Words

Strings

String Methods

Numbers

Arithmetic Operations

Number Methods

Conditionals

if switch

Loops

while do for

Functions

A function is generally a named section of code that can be “called” by name, and the code inside it will be run each time.
Functions can optionally take arguments (aka parameters)—values you pass in. And they can also optionally return a value back.

Scope

Scope consists of a series of "bubbles" that each act as a container or bucket, in which identifiers (variables, functions) are declared. These bubbles nest neatly inside each other, and this nesting is defined at author-time.
The lexing phase of compilation is essentially able to know where and how all identifiers are declared, and thus predict how they will be looked-up during execution.
Two mechanisms in JavaScript can "cheat" lexical scope: eval(..) and with.
The Principle of Least Privilege states that in the design of software, such as the API for a module/object, you should expose only what is minimally necessary, and "hide" everything else.

Closure

Hoisting

Prototype

Notes

JavaScript was created in 1995 by Netscape
There’s no need to actually use a semicolon to terminate a statement because JavaScript interpreters use a process called Automatic Semicolon Insertion (ASI).
Technically, only objects have properties and methods. JavaScript overcomes this by creating wrapper objects for primitive values. This all happens in the back- ground, so for all intents and purposes it appears that primitive values also have properties and methods.
TODO IDEs
TODO Debugging
Console

Sources

You Don't Know Javascript by Kyle Simpson
MDN

jorsh/lean-javascript