let

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

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Syntax

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

Parameters

var1, var2, …, varN

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

value1, value2, …, valueN Optional

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

Description

let allows you to declare variables that are limited to a scope of a block statement, or expression on which it is used, unlike the var keyword, which defines 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.

Scoping rules

Variables declared by let have their scope in the block for 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 = 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

Unlike variables declared with var, which will start with the value undefined, let variables are not initialized until their definition is evaluated. Accessing the variable before the initialization results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.

function do_something() {
  console.log(bar); // undefined
  console.log(foo); // ReferenceError
  var bar = 1;
  let foo = 2;
}

The temporal dead zone and typeof

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 temporal dead zone will throw a ReferenceError:

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

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

Another example of temporal dead zone combined with lexical scoping

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 the same 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). 

The if block's foo is 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. 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

Browser compatibility

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?