JS: Variable

By Xah Lee. Date: . Last updated: .

for JavaScript code after ECMAScript 2015, you need to declare variable.

// declare a variable and assign a value
let x = 4;
console.log(x);
// 4
// If a variable is not assigned, it has a value of undefined
let x;
console.log(x === undefined);

Historical note

in JavaScript before ECMAScript 2015, variable do not need to be declared. You can simply assign it e.g. x = 3. It becomes property of the Global Object .

After ECMAScript 2015 , variable needs to be declared, by using let or const.

In Strict Mode, variable also need to be declared.