let

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

Синтаксис

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

Параметры

var1, var2, …, varN

Имя переменной. Может использоваться любой допустимый идентификатор.

value1, value2, …, valueN

Значение переменной. Любое допустимое выражение.

Описание

Позволяет объявлять переменные которые ограничены областью дейсвтия блока, statement, или выражения в котором они используются. В отличие от ключевого слова var, которое объявляет переменную глобально или локально во всей функции независимо от области блока.

Область блока с 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

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

Browser compatibility

Firefox-specific notes

• [1]: Only available to code blocks in HTML wrapped in a