nameN

The name of the variable to declare. Each must be a legal JavaScript identifier or a destructuring binding pattern.

valueN Optional

Initial value of the variable. It can be any legal expression. Default value is undefined.

The scope of a variable declared with let is one of the following curly-brace-enclosed syntaxes that most closely contains the let declaration:

• Block statement
• switch statement
• try...catch statement
• Body of one of the for statements, if the let is in the header of the statement
• Function body
• Static initialization block

Or if none of the above applies:

• The current module, for code running in module mode
• The global scope, for code running in script mode.

Compared with var, let declarations have the following differences:

• let declarations are scoped to blocks as well as functions.
• let declarations can only be accessed after the place of declaration is reached (see temporal dead zone). For this reason, let declarations are commonly regarded as non-hoisted.
• let declarations do not create properties on globalThis when declared at the top level of a script.
• let declarations cannot be redeclared by any other declaration in the same scope.
• let begins declarations, not statements. That means you cannot use a lone let declaration as the body of a block (which makes sense, since there's no way to access the variable).
  js

  if (true) let a = 1; // SyntaxError: Lexical declaration cannot appear in a single-statement context
  

Note that let is allowed as an identifier name when declared with var or function in non-strict mode, but you should avoid using let as an identifier name to prevent unexpected syntax ambiguities.

Many style guides (including MDN's) recommend using const over let whenever a variable is not reassigned in its scope. This makes the intent clear that a variable's type (or value, in the case of a primitive) can never change. Others may prefer let for non-primitives that are mutated.

The list that follows the let keyword is called a binding list and is separated by commas, where the commas are not comma operators and the = signs are not assignment operators. Initializers of later variables can refer to earlier variables in the list.

A variable declared with let, const, or class is said to be in a "temporal dead zone" (TDZ) from the start of the block until code execution reaches the place where the variable is declared and initialized.

While inside the TDZ, the variable has not been initialized with a value, and any attempt to access it will result in a ReferenceError. The variable is initialized with a value when execution reaches the place in the code where it was declared. If no initial value was specified with the variable declaration, it will be initialized with a value of undefined.

This differs from var variables, which will return a value of undefined if they are accessed before they are declared. The code below demonstrates the different result when let and var are accessed in code before the place where they are declared.

{
  // TDZ starts at beginning of scope
  console.log(bar); // "undefined"
  console.log(foo); // ReferenceError: Cannot access 'foo' before initialization
  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!
}

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

typeof i; // ReferenceError: Cannot access 'i' before initialization
let i = 10;

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

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

let declarations cannot be in the same scope as any other declaration, including let, const, class, function, var, and import declaration.

{
  let foo;
  let foo; // SyntaxError: Identifier 'foo' has already been declared
}

A let declaration within a function's body cannot have the same name as a parameter. A let declaration within a catch block cannot have the same name as the catch-bound identifier.

function foo(a) {
  let a = 1; // SyntaxError: Identifier 'a' has already been declared
}
try {
} catch (e) {
  let e; // SyntaxError: Identifier 'e' has already been declared
}

If you're experimenting in a REPL, such as the Firefox web console (Tools > Web Developer > Web Console), and you run two let declarations with the same name in two separate inputs, you may get the same re-declaration error. See further discussion of this issue in Firefox bug 1580891. The Chrome console allows let re-declarations between different REPL inputs.

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: Identifier 'foo' has already been declared
    break;
}

To avoid the error, wrap each case in a new block statement.

let x = 1;

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

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

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 initialization 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 scope of the for...of 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); // { 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;

{
  var a = 11; // the scope is global
  let b = 22; // the scope is inside the block

  console.log(a); // 11
  console.log(b); // 22
}

console.log(a); // 11
console.log(b); // 2

However, this combination of var and let declarations below is a SyntaxError because var not being block-scoped, leading to them being in the same scope. This results in an implicit re-declaration of the variable.

let x = 1;

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

The left-hand side of each = can also be a binding pattern. This allows creating multiple variables at once.

const result = /(a+)(b+)(c+)/.exec("aaabcc");
let [, a, b, c] = result;
console.log(a, b, c); // "aaa" "b" "cc"

For more information, see Destructuring assignment.

• var
• const
• Hoisting
• ES6 In Depth: let and const on hacks.mozilla.org (2015)
• Breaking changes in let and const in Firefox 44 on blog.mozilla.org (2015)
• You Don't Know JS: Scope & Closures, Ch.3: Function vs. Block Scope by Kyle Simpson
• What is the Temporal Dead Zone? on Stack Overflow
• What is the difference between using let and var? on Stack Overflow
• Why was the name 'let' chosen for block-scoped variable declarations in JavaScript? on Stack Overflow