Non-standard

Introduced in JavaScript 1.7

Summary

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

Syntax

let definition:

let var1 [= value1] [, var2 [= value2]] [, ..., varN [= valueN]];

let expression:

let (var1 [= value1] [, var2 [= value2]] [, ..., varN [= valueN]]) expression;

let statement:

let (var1 [= value1] [, var2 [= value2]] [, ..., varN [= valueN]]) statement;

Parameters

Description

let allows you to declare variables, limiting its scope to the block, statement, or expression on which is used. This is unlike the var keyword, which defines a variable globally, or local to an entire function regardless of block scope.

Examples

A let expression limit the scope of the variable declared only in that expression.

var a = 5;
let(a = 6) alert(a); // 6
alert(a); // 5

 Used inside a block, let limits the variable's scope to that block. Note the difference between var which its scope is inside the function where is declared

var a = 5;
var b = 10;
if (a === 5) {
  let a = 4; // The scope is inside the if-block
  var b = 1; // The scope is inside the function

  console.log(a);  // 4
  console.log(b);  // 1
} 
console.log(a); // 5
console.log(b); // 1

You can use the let keyword to bind variables locally in the scope of for loops instead of using a global variable (defined using var) for that.

for (let i = 0; i<10; i++) {
  alert(i); // 1, 2, 3, 4 ... 9
}

alert(i); // i is not defined

See also

• var
• const
• New in JavaScript 1.7