This is a new technology, part of the ECMAScript 2015 (ES6) standard.
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.

The let statement declares a block scope local variable, optionally initializing it to a value.

The let keyword in Mozilla Firefox is only available to code blocks in HTML wrapped in a <script type="application/javascript;version=1.7"> block (or higher version). XUL script tags have access to these features without needing this special block. Beware, however, that as this is a non-standard feature, this will most likely break support for other browsers.

Syntax

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

Parameters

var1, var2, …, varN
Variable name. It can be any legal identifier.
value1, value2, …, valueN
Initial value of the variable. It can be any legal expression.

Description

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.

Block scope with let

Use the let keyword to define variables inside a block.

if (x > y) {
  let gamma = 12.7 + y;
  i = gamma * x;
}

let sometimes makes the code cleaner when inner functions are used.

var list = document.getElementById("list");

for (var i = 1; i <= 5; i++) {
  var item = document.createElement("LI");
  item.appendChild(document.createTextNode("Item " + i));

  let j = i;
  item.onclick = function (ev) {
    console.log("Item " + j + " is clicked.");
  };
  list.appendChild(item);
}

The example above works as intended because the five instances of the (anonymous) inner function refer to five different instances of variable j. Note that it does not work as intended if you replace let by var or if you remove the variable j and simply use the variable i in the inner function.

Scoping rules

Variables declared by let have as their scope the block in which they are defined, as well as in any contained sub-blocks . In this way, let works very much like var. The main difference is that the scope of a var variable is the entire enclosing function:

function varTest() {
  var x = 31;
  if (true) {
    var x = 71;  // same variable!
    console.log(x);  // 71
  }
  console.log(x);  // 71
}

function letTest() {
  let x = 31;
  if (true) {
    let x = 71;  // different variable
    console.log(x);  // 71
  }
  console.log(x);  // 31
}

At the top level of programs and functions, let behaves exactly like var does. For example:

var x = 'global';
let y = 'global';
console.log(this.x);
console.log(this.y);

The output displayed by this code will display "global" twice.

Temporal dead zone and errors with let

Redeclaration of the same variable in the same function or block scope raises a TypeError.

if (x) {
  let foo;
  let foo; // TypeError thrown.
}

In ECMAScript 6, let does not hoist the variable to the top of the block. If you reference a variable in a block before the let declaration for that variable is encountered, this results in a ReferenceError, because the variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

function do_something() {
  console.log(foo); // ReferenceError
  let foo = 2;
}

You may encounter errors in switch statements because there is only one underlying block.

switch (x) {
  case 0:
    let foo;
    break;
    
  case 1:
    let foo; // TypeError for redeclaration.
    break;
}

let-scoped variables in for loops

You can use the let keyword to bind variables locally in the scope of for loops. This is different from the var keyword in the head of a for loop, which makes the variables visible in the whole function containing the loop.

var i=0;
for ( let i=i ; i < 10 ; i++ ) {
  console.log(i);
}

Scoping rules

for (let expr1; expr2; expr3) statement

In this example, expr2, expr3, and statement are enclosed in an implicit block that contains the block local variables declared by let expr1. This is demonstrated in the first loop above.

Examples

let vs var

When used inside a block, let limits the variable's scope to that block. Note the difference between var whose scope is inside the function where it 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

let in loops

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

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

console.log(i); // i is not defined

Non-standard let extensions

The let block and let expression syntax is non-standard and will be removed in the future. Do not use them! See bug 1023609 and bug 1167029 for more details.

let blocks

The let block provides a way to associate values with variables within the scope of a block, without affecting the values of like-named variables outside the block.

Syntax

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

Description

The let block provides local scoping for variables. It works by binding zero or more variables in the lexical scope of a single block of code; otherwise, it is exactly the same as a block statement. Note in particular that the scope of a variable declared inside a let block using var is still the same as if it had been declared outside the let block; such variables still have function scoping. When using the let block syntax, the parentheses following let are required. Failure to include them will result in a syntax error.

Example

var x = 5;
var y = 0;

let (x = x+10, y = 12) {
  console.log(x+y); // 27
}

console.log(x + y); // 5

The rules for the code block are the same as for any other code block in JavaScript. It may have its own local variables established using the let declarations.

Scoping rules

The scope of variables defined using let is the let block itself, as well as any inner blocks contained inside it, unless those blocks define variables by the same names.

let expressions

let expression support has been dropped in Gecko 41 (bug 1023609).

The let expression lets you establish variables scoped only to a single expression.

Syntax

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

Example

You can use let to establish variables that are scoped only to a single expression:

var a = 5;
let(a = 6) console.log(a); // 6
console.log(a); // 5

Scoping rules

Given a let expression:

let (decls) expr

There is an implicit block created around expr.

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Standard Initial definition. Does not specify let expressions or let statements.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support

41.0

2.0 (1.8.1) 11 17 ?
Temporal dead zone ? 35 (35) ? ? ?
let expression Not supported Not supported Not supported Not supported Not supported
let block Not supported 2.0 (1.8.1) Not supported Not supported Not supported
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ?

41.0

1.0 (1.8.1) ? ? ?
Temporal dead zone ? ? 35.0 (35) ? ? ?
let expression Not supported Not supported Not supported Not supported Not supported Not supported
let block Not supported Not supported 1.0 (1.8.1) Not supported Not supported Not supported

Firefox-specific notes

  • Only available to code blocks in HTML wrapped in a <script type="application/javascript;version=1.7"> block (or higher version). Beware, however, that as this is a non-standard feature, this will most likely break support for other browsers. XUL script tags have access to these features without needing this special block. See bug 932517 and bug 932513.
  • Support in Worker code is hidden behind the dom.workers.latestJSVersion flag. bug 487070
  • ES6 compliance for let in SpIderMonkey is tracked in bug 950547 and non-standard extensions are going to be removed in the future bug 1023609.

See also

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