The let statement declares a block scope local variable, optionally initializing it to a value.
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.
An explanation of why the name "let" was chosen can be found here.
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 = 1;
if (true) {
var x = 2; // same variable!
console.log(x); // 2
}
console.log(x); // 2
}
function letTest() {
let x = 1;
if (true) {
let x = 2; // different variable
console.log(x); // 2
}
console.log(x); // 1
}
Cleaner code in inner functions
let sometimes makes the code cleaner when inner functions are used.
var list = document.getElementById("list");
for (let i = 1; i <= 5; i++) {
let item = document.createElement("li");
item.appendChild(document.createTextNode("Item " + i));
item.onclick = function (ev) {
console.log("Item " + i + " 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 the variable i. Note that it does not work as intended if you replace let with var, since all of the inner functions would then return the same final value of i: 6. Also, we can keep the scope around the loop cleaner by moving the code that creates the new elements into the scope of each loop.
At the top level of programs and functions, let, unlike var, does not create a property on the global object. For example:
var x = 'global'; let y = 'global'; console.log(this.x); // "global" console.log(this.y); // undefined
Emulating private interfaces
In dealing with constructors it is possible to use the let statement in order to create a private interface without using closures:
var SomeConstructor;
{
let privateScope = {};
SomeConstructor = function SomeConstructor () {
this.someProperty = "foo";
privateScope.hiddenProperty = "bar";
}
SomeConstructor.prototype.showPublic = function () {
console.log(this.someProperty); // foo
}
SomeConstructor.prototype.showPrivate = function () {
console.log(privateScope.hiddenProperty); // bar
}
}
var myInstance = new SomeConstructor();
myInstance.showPublic();
myInstance.showPrivate();
console.log(privateScope.hiddenProperty); // error
Temporal dead zone and errors with let
Redeclaring the same variable within the same function or block scope raises a {{jsxref("SyntaxError")}}.
if (x) {
let foo;
let foo; // SyntaxError thrown.
}
In ECMAScript 2015, let will hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError. 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; // SyntaxError for redeclaration.
break;
}
Using let with a variable name that is the same as a parameter passed to a function will result in undefined inside a for loop.
function go(n){
for (let n of n.a) {
console.log(n);
}
}
go({a:[1,2,3]});
Another example
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 = 1;
var b = 2;
if (a === 1) {
var a = 11; // the scope is global
let b = 22; // the scope is inside the if-block
console.log(a); // 11
console.log(b); // 22
}
console.log(a); // 11
console.log(b); // 2
Specifications
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ES6', '#sec-let-and-const-declarations', 'Let and Const Declarations')}} | {{Spec2('ES6')}} | Initial definition. Does not specify let expressions or let blocks. |
| {{SpecName('ESDraft', '#sec-let-and-const-declarations', 'Let and Const Declarations')}} | {{Spec2('ESDraft')}} |
Browser compatibility
{{CompatibilityTable}}
| Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
| Basic support | {{CompatChrome(41.0)}} | 12 | {{ CompatGeckoDesktop(44) }} | 11 | 17 | {{CompatUnknown}} |
| Temporal dead zone | {{CompatUnknown}} | 12 | {{ CompatGeckoDesktop(35) }} | 11 | {{CompatUnknown}} | {{CompatUnknown}} |
| Allowed in {{Glossary("sloppy mode")}} | {{CompatChrome(49.0)}} | {{CompatUnknown}} | {{CompatGeckoDesktop(44)}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} |
| Feature | Android | Android Webview | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile | Chrome for Android |
|---|---|---|---|---|---|---|---|
| Basic support | {{CompatUnknown}} | {{CompatChrome(41.0)}} | {{ CompatGeckoMobile(44) }} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatChrome(41.0)}} |
| Temporal dead zone | {{CompatUnknown}} | {{CompatUnknown}} | {{ CompatGeckoMobile(35) }} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} |
| Allowed in {{Glossary("sloppy mode")}} | {{CompatNo}} | {{CompatChrome(49.0)}} | {{CompatGeckoDesktop(44)}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatChrome(49.0)}} |
Firefox-specific notes
- Prior to SpiderMonkey 46 {{geckoRelease(46)}}, a {{jsxref("TypeError")}} was thrown on redeclaration instead of a {{jsxref("SyntaxError")}} ({{bug(1198833)}}, {{bug(1275240)}}).
- Prior to SpiderMonkey 44 {{geckoRelease(44)}},
letwas only available to code blocks in HTML wrapped in a<script type="application/javascript;version=1.7">block (or higher version) and had different semantics. - Support in {{domxref("Worker")}} code is hidden behind the
dom.workers.latestJSVersionflag ({{bug(487070)}}). With version freelet, this flag is going to be removed in the future ({{bug(1219523)}}).