JavaScript Variables & Datatypes¶

Datatypes¶

var x; // undefined
var x = 5; // number
var x = 3.14; // number
var x = "Test"; // string
var x = ["Test", "Test2"]; // array
var x = true; // boolean
var x = null; // null

Objects¶

// object
var object = {
    key1 "value1",
    key2: 12,
    total: function() {
        return this.key1 + this.key2;
    }
};

object.key1 = "new value"; // change value
object[key2]++; // increment key2
total = object.total(); // call function

Variable Scopes¶

type	scope	hoisting	hoisting init
var	global/function/local	top of scope	`undefined`
let	block	top of scope
const	block	top of scope

var¶

var can be redeclared and updated (global/function/local scoped)

var x = 10; // var Variable

function test() {
    var y = 20;
    var x = y;
    console.log(x); // 20
}

let¶

let can't be redeclared but updated (block scoped)

let x = 10; // let Variable

let x = 20; // error can't redeclare
x = 20; // update

function test() {
    let x = 20; // no error because function scoped
}

const¶

const can't be redeclared or updated (block scoped)

const x = 10; // const Variable

const x = 20; // error can't redeclare
x = 20; // error can't update

function test() {
    const x = 20; // no error because function scoped
}

Objects and Arrays can be updated

const object = {
    key1: "value1",
    key2: 12,
};

object.key1 = "new value"; // no error