JavaScript Variables and Data Types

Keyword	Scope	Re-assign?	Re-declare?	Hoisted?
var	Function	Yes	Yes	Yes (as undefined)
let	Block	Yes	No	Yes, but in the temporal dead zone
const	Block	No	No	Yes, but in the temporal dead zone

Modern ruleUse const by default, let when the value must change, and avoid var.

Data Types

JavaScript

let n = 42;               // number
let big = 9007199254740993n; // bigint
let s = "hello";          // string
let ok = true;            // boolean
let nothing = null;       // null
let notSet;               // undefined
let id = Symbol("id");    // symbol
let obj = { a: 1 };       // object (arrays and functions are objects too)

console.log(typeof n, typeof s, typeof nothing, typeof notSet, typeof obj);

number string object undefined object

Famous quirktypeof null returns "object". It's a bug from the first version of JavaScript that was kept for compatibility.

Template Literals

JavaScript

const name = "Asha", marks = 88;
console.log(`${name} scored ${marks} (${marks >= 40 ? "Pass" : "Fail"})`);

Asha scored 88 (Pass)

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