let - JavaScript 编辑

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

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

let name1 [= value1] [, name2 [= value2]] [, ..., nameN [= valueN];

Parameters

nameN

The names of the variable or variables to declare. Each must be a legal JavaScript identifier.

valueN Optional

For each variable declared, you may optionally specify its initial value to any legal JavaScript expression.

 Alternatively, the Destructuring Assignment syntax can also be used to declare variables.

let { bar } = foo; // where foo = { bar:10, baz:12 };
/* This creates a variable with the name 'bar', which has a value of 10 */

Description

let allows you to declare variables that are limited to the scope of a block statement, or expression on which it is used, unlike the var keyword, which declares a variable globally, or locally to an entire function regardless of block scope. The other difference between var and let is that the latter is initialized to a value only when a parser evaluates it (see below).

Just like const the let does not create properties of the window object when declared globally (in the top-most scope).

An explanation of why the name "let" was chosen can be found here.

Many issues with let variables can be avoided by declaring them at the top of the scope in which they are used (doing so may impact readibility).

Examples

Scoping rules

Variables declared by let have their scope in the block for which they are declared, 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;
  {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}

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 members

In dealing with constructors it is possible to use the let bindings to share one or more private members without using closures:

var Thing;

{
  let privateScope = new WeakMap();
  let counter = 0;

  Thing = function() {
    this.someProperty = 'foo';

    privateScope.set(this, {
      hidden: ++counter,
    });
  };

  Thing.prototype.showPublic = function() {
    return this.someProperty;
  };

  Thing.prototype.showPrivate = function() {
    return privateScope.get(this).hidden;
  };
}

console.log(typeof privateScope);
// "undefined"

var thing = new Thing();

console.log(thing);
// Thing {someProperty: "foo"}

thing.showPublic();
// "foo"

thing.showPrivate();
// 1

The same privacy pattern with closures over local variables can be created with var, but those need a function scope (typically an IIFE in the module pattern) instead of just a block scope like in the example above.

Redeclarations

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

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

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

let x = 1;
switch(x) {
  case 0:
    let foo;
    break;

  case 1:
    let foo; // SyntaxError for redeclaration.
    break;
}

However, it's important to point out that a block nested inside a case clause will create a new block scoped lexical environment, which will not produce the redeclaration errors shown above.

let x = 1;

switch(x) {
  case 0: {
    let foo;
    break;
  }
  case 1: {
    let foo;
    break;
  }
}

Temporal dead zone (TDZ)

let variables cannot be read/written until they have been fully initialized, which happens when they are declared (if no initial value is specified on declaration, the variable is initialized with a value of undefined). Accessing the variable before the initialization results in a ReferenceError.

This differs from var variables, which will return a value of undefined if they are accessed before they are declared.

The variable is said to be in a "temporal dead zone" (TDZ) from the start of the block until the initialization has completed.

{ // TDZ starts at beginning of scope
  console.log(bar); // undefined
  console.log(foo); // ReferenceError
  var bar = 1;
  let foo = 2; // End of TDZ (for foo)
}

The term "temporal" is used because the zone depends on the order of execution (time) rather than the order in which the code is written (position). For example, the code below works because, even though the function that uses the let variable appears before the variable is declared, the function is called outside the TDZ.

{
    // TDZ starts at beginning of scope
    const func = () => console.log(letVar); // OK

    // Within the TDZ letVar access throws `ReferenceError`

    let letVar = 3; // End of TDZ (for letVar)
    func(); // Called outside TDZ!
}

The TDZ and typeof

Using the typeof operator for a let variable in its TDZ will throw a ReferenceError:

// results in a 'ReferenceError'
console.log(typeof i);
let i = 10;

This differs from using typeof for undeclared variables, and variables that hold a value of undefined:

// prints out 'undefined'
console.log(typeof undeclaredVariable);

TDZ combined with lexical scoping

The following code results in a ReferenceError at the line shown:

function test(){
   var foo = 33;
   if(foo) {
      let foo = (foo + 55); // ReferenceError
   }
}
test();

The if block is evaluated because the outer var foo has a value. However due to lexical scoping this value is not available inside the block: the identifier foo inside the if block is the let foo. The expression (foo + 55) throws a ReferenceError because initialisation of let foo has not completed — it is still in the temporal dead zone.

This phenomenon can be confusing in a situation like the following. The instruction let n of n.a is already inside the private scope of the for loop's block. So, the identifier n.a is resolved to the property 'a' of the 'n' object located in the first part of the instruction itself (let n).

This is still in the temporal dead zone as its declaration statement has not been reached and terminated.

function go(n) {
  // n here is defined!
  console.log(n); // Object {a: [1,2,3]}

  for (let n of n.a) { // ReferenceError
    console.log(n);
  }
}

go({a: [1, 2, 3]});

Other situations

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

However, this combination of var and let declaration below is a SyntaxError due to var being hoisted to the top of the block. This results in an implicit re-declaration of the variable.

let x = 1;

{
  var x = 2; // SyntaxError for re-declaration
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Let and Const Declarations' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• var
• const
• ES6 In Depth: let and const
• Breaking changes in let and const in Firefox 44
• You Don't Know JS: Scope & Closures: Chapter 3: Function vs. Block Scope
• StackOverflow: What is the Temporal Dead Zone?
• StackOverflow: What is the difference between using let and var?