let
The let
declaration declares re-assignable, block-scoped local variables, optionally initializing each to a value.
Try it
Syntax
let name1;
let name1 = value1;
let name1 = value1, name2 = value2;
let name1, name2 = value2;
let name1 = value1, name2, /* …, */ nameN = valueN;
Parameters
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
.
Description
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
statementtry...catch
statement- Body of one of the
for
statements, if thelet
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 onglobalThis
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 lonelet
declaration as the body of a block (which makes sense, since there's no way to access the variable).jsif (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.
Temporal dead zone (TDZ)
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 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"
Note: let
and const
declarations are only processed when the current script gets processed. If you have two <script>
elements running in script mode within one HTML, the first script is not subject to the TDZ restrictions for top-level let
or const
variables declared in the second script, although if you declare a let
or const
variable in the first script, declaring it again in the second script will cause a redeclaration error.
Redeclarations
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;
}
}
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
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 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] });
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;
{
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
}
Declaration with destructuring
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.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-let-and-const-declarations |
Browser compatibility
BCD tables only load in the browser
See also
var
const
- Hoisting
- ES6 In Depth:
let
andconst
on hacks.mozilla.org (2015) - Breaking changes in
let
andconst
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
andvar
? on Stack Overflow - Why was the name 'let' chosen for block-scoped variable declarations in JavaScript? on Stack Overflow