let

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Příkaz let deklaruje lokální proměnnou, jejíž rozsah je omezen na aktuální blok a volitelně ji inicializuje na danou hodnotu.

Syntaxe

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

Parametry

var1, var2, …, varN
Název proměnné. Může to být jakýkoliv platný identifikátor.
value1, value2, …, valueN
Výchozí hodnota proměnné. Může to být jakýkoliv platný výraz.

Popis

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, unlike var, does not create a property on the global object. 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" for this.x but undefined for this.y.

Temporal dead zone and errors with let

Redeclaring the same variable within the same function or block scope raises a TypeError.

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

Příklady

let versus 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

let blocks

let blocks support has been dropped in Gecko 44 (chyba 1167029).

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 (chyba 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 blocks.
ECMAScript 2016 Draft (7th Edition, ECMA-262)
The definition of 'Let and Const Declarations' in that specification.
Draft  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 41.0 12 44 (44) 11 17 ?
Temporal dead zone ? 12 35 (35) 11 ? ?
let expression No support No support No support No support No support No support
let block No support No support No support No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 41.0 44.0 (44) ? ? ?
Temporal dead zone ? ? 35.0 (35) ? ? ?
let expression No support No support No support No support No support No support
let block No support No support No support No support No support No support

Firefox-specific notes

  • Prior to SpiderMonkey 44 (Firefox 44 / Thunderbird 44 / SeaMonkey 2.41), let was 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 Worker code is hidden behind the dom.workers.latestJSVersion flag (chyba 487070). With version free let, this flag is going to be removed in the future (chyba 1219523).
  • ES6 compliance for let in SpIderMonkey is tracked in chyba 950547

See also

Document Tags and Contributors

 Contributors to this page: babant
 Last updated by: babant,