let

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

var1, var2, …, varN

變數名稱。

value1, value2, …, valueN

變數的初始值，可以是任何合法的表達式。

let 可以宣告只能在目前區塊、階段或表達式中作用的變數。而 var 則是定義了一個全域變數，或是在整個 function 而不管該區塊範圍。

宣告 let 的作用範圍是它們被定義的區塊，以及該區塊包含的子區塊。這樣看起來功能跟 var 很相似。主要不同的地方在於 var 作用範圍是「整個」function：

function varTest() {
  var x = 1;
  {
    var x = 2; // 這裡的 x 與 function 區塊內部的 x 是一樣的，因此會影響 function 區塊內所有的 x
    console.log(x); // 2
  }
  console.log(x); // 2
}

function letTest() {
  let x = 1;
  {
    let x = 2; // 這裡的 x 與 function 區塊內部的 x 是不同的，只會作用在這層 block 區塊中
    console.log(x); // 2
  }
  console.log(x); // 1
}

在上列例子裡的最前行 let 和 var 不同，let 並不會在全域物件中建立變數。舉例來說：

var x = "global";
let y = "global";
console.log(this.x); // "global"
console.log(this.y); // undefined

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

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

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

In ECMAScript 2015, let bindings are not subject to Variable Hoisting, which means that let declarations do not move to the top of the current execution context. Referencing the variable in the block before the initialization results in a ReferenceError (contrary to a variable declared with var, which will just have the undefined value). The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.

function do_something() {
  console.log(foo); // ReferenceError
  let foo = 2;
}

你可能會在 switch 中遇到錯誤，因為所有的 case 都屬於同樣的區塊中。

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

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

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);
}

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;
  }
}

Unlike with simply undeclared variables and variables that hold a value of undefined, using the typeof operator to check for the type of a variable in that variable's TDZ will throw a ReferenceError:

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

Due to lexical scoping, the identifier "foo" inside the expression (foo + 55) evaluates to the if block's foo, and not the overlying variable foo with the value of 33. In that very line, the if block's "foo" has already been created in the lexical environment, but has not yet reached (and terminated) its initialization (which is part of the statement itself): it's still in the temporal dead zone.

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

This phenomenon may confuse you 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, hence 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"), which is still in the temporal dead zone since 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] });

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

• var
• const
• ES6 In Depth: let and const
• Breaking changes in let and const in Firefox 44.