variables.md

ES6 Variables

var vs let

A variable in javascript is a named space in memory. Traditionally the keyword var initializes the identifier with a value:

var	my_variable	=	'value';
//1	//2			//3 

//1 the var keyword
//2 the identifier
//3 the value

There are rules for naming the variable identifier. These are:

• identifiers cannot be keywords
• can be alphanumeric, although cannot start with a number
• $ and _ are also allowed characters for an identifier

Variables declared by var have the scope of the entire function.

function myFunc() {
	if(true) {
		var my_var = 'test';
	}
	console.log(my_var); // test
}

The let keyword

let is preferred over var. Variables declared by let have their scope within the block they are defined.

function myFunc() {
	if(true) {
		let my_var = 'test';
	}
	console.log(my_var); // TypeError
}

Block scoping allows for variable shadowing.

function myFunc() {
	let my_var = 'test';
	if(true) {
		let my_var = 'new test';
		console.log(my_var); // new test
	}
	console.log(my_var); // test
}

The const keyword

ES6 also introduced a new variable keyword: const. Variables declared with the const keyword are block scoped just like let however they cannot change by reassignment and they cannot be re-declared; they are immutable.

const version = '0.0.1';
version = '0.0.2'; // TypeError: invalid assignment to const

const name = 'bill';
const name = 'ted'; // SyntaxError: Identifier 'name' has already been declared

Variables declared by const (constants) cannot be changed. However, with a for loop the scope is redeclared at the start of each loop, where a new const can be initalized.

function myFunc(items) {
	for(let i = 0; i < items.length; i++) {
		const message = items[i] + ' found at index: ' + i;
		console.log(message);
	} 
}

myFunc(['test', 100, 200]);