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    let

    这篇翻译不完整。请帮忙从英语翻译这篇文章

    该特性处于 ECMAScript 6 规范草案中,目前的实现在未来可能会发生微调,请谨慎使用。

    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 and bug 1167029 for more details.

    let blocks

    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

    let expression support has been dropped in Gecko 41 (bug 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 6 (ECMA-262)
    Let and Const Declarations
    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 未实现 2.0 (1.8.1)-40 (40) [1] 未实现 未实现 未实现
    let block 未实现 2.0 (1.8.1) [1] 未实现 未实现 未实现
    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 未实现 未实现 1.0 (1.8.1)-40.0 (40)[1] 未实现 未实现 未实现
    let block 未实现 未实现 1.0 (1.8.1) [1] 未实现 未实现 未实现

    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.

    See also

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    Contributors to this page: teoli, ziyunfei, Nightire, SphinxKnight, ted423, Junjie_Wei, WangZishi
    最后编辑者: ziyunfei,
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