This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal.
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.
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; }
You can use let
definitions to alias pseudo-namespaced code in extensions. (See Security best practices in extensions.)
let Cc = Components.classes, Ci = Components.interfaces;
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 block scope raises a TypeError
.
if (x) { let foo; let foo; // TypeError thrown. }
However, function bodies do not have this limitation!
function do_something() { let foo; let foo; // This works fine. }
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 for more details.
let
block
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]]) statement;
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
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 6 (ECMA-262) The definition of 'Let and Const Declarations' in that specification. |
Release Candidate | 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) [1] | 11 | 17 | ? |
Temporal dead zone | ? | 35 (35) [1] | ? | ? | ? |
let expression |
Not supported | 2.0 (1.8.1) [1] | Not supported | Not supported | Not supported |
let block |
Not supported | 2.0 (1.8.1) [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) [1] | ? | ? | ? |
Temporal dead zone | ? | ? | 35.0 (35) [1] | ? | ? | ? |
let expression |
Not supported | Not supported | 1.0 (1.8.1) [1] | Not supported | Not supported | Not supported |
let block |
Not supported | Not supported | 1.0 (1.8.1) [1] | Not supported | Not supported | Not supported |
Firefox-specific notes
- [1]: 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. - 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.