JavaScript Let
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JavaScript Let
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ECMAScript 2015
ES2015 introduced two important new JavaScript keywords: let and const.
These two keywords provide Block Scope variables (and
constants) in JavaScript.
Before ES2015, JavaScript had only two types of scope: Global Scope
and Function Scope.
Global Scope
Variables declared Globally (outside any function) have
Global Scope.
Example
var carName = "Volvo";
// code here can use carName
function myFunction()
// code here can also use carName
Try it Yourself »
Global variables can be accessed from anywhere in a JavaScript program.
Function Scope
Variables declared Locally (inside a function) have
Function Scope.
Example
// code here can NOT use carName
function myFunction()
var carName = "Volvo";
// code here CAN use carName
// code here can NOT use carName
Try it Yourself »
Local variables can only be accessed from inside the function where they are
declared.
JavaScript Block Scope
Variables declared with the var keyword can not have Block
Scope.
Variables declared inside a block can be accessed from
outside the block.
Example
var x = 2;
// x CAN be used hereBefore ES2015 JavaScript did not have Block Scope.
Variables declared with the let keyword can have Block Scope.
Variables declared inside a block can not be accessed
from outside the block:
Example
let x = 2;
// x can NOT be used hereRedeclaring Variables
Redeclaring a variable using the var keyword can impose problems.
Redeclaring a variable inside a block will also redeclare the variable
outside the block:
Example
var x = 10;
// Here x is 10
var x = 2;
// Here x is 2
// Here x is 2Try it Yourself »
Redeclaring a variable using the let keyword can solve this problem.
Redeclaring a variable inside a block will not redeclare the variable outside
the block:
Example
var x = 10;
// Here x is 10
let x = 2;
// Here x is 2
// Here x is 10
Try it Yourself »
Browser Support
The let keyword is not fully supported in Internet Explorer 11 or earlier.
The following table defines the first browser versions with full support for the let keyword:
| Chrome 49 | IE / Edge 12 | Firefox 44 | Safari 11 | Opera 36 |
| Mar, 2016 | Jul, 2015 | Jan, 2015 | Sep, 2017 | Mar, 2016 |
Loop Scope
Using var in a loop:
Example
var i = 5;
for (var i = 0; i < 10; i++)
// some statements
// Here i is 10
Try it Yourself »
Using let in a loop:
Example
let i = 5;
for (let i = 0; i < 10; i++)
// some statements
// Here i is 5
Try it Yourself »
In the first example, using var, the variable declared in
the loop redeclares the variable outside the loop.
In the second example, using let, the variable declared in
the loop does not redeclare the variable outside the loop.
When let is used to declare the i variable in a loop, the i
variable will only be visible within the loop.
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Function Scope
Variables declared with var and let are
quite similar when declared inside a function.
They will both have Function Scope:
function myFunction()
var carName = "Volvo";
// Function Scope
function myFunction()
let carName = "Volvo"; //
Function Scope
Global Scope
Variables declared with var and let are
quite similar when declared outside a block.
They will both have Global Scope:
var x = 2;
// Global scope
let x = 2; //
Global scopeGlobal Variables in HTML
With JavaScript, the global scope is the JavaScript environment.
In HTML, the global scope is the window object.
Global variables defined with the var keyword belong to the window object:
Example
var carName = "Volvo";
// code here can use window.carName
Try it Yourself »
Global variables defined with the let keyword do not belong to the window object:
Example
let carName = "Volvo";
// code here can not use window.carName
Try it Yourself »
Redeclaring
Redeclaring a JavaScript variable with var is allowed
anywhere in a program:
Example
var x = 2;
//
Now x is 2
var x = 3;
// Now x is 3Try it Yourself »
Redeclaring a var variable with let, in the same scope, or in
the same block, is not allowed:
Example
var x = 2; // Allowed
let x = 3; // Not allowed
var x = 4; // Allowed
let x = 5 // Not allowed
Redeclaring a variable with let, in the same scope, or in
the same block, is not allowed:
Example
let x = 2; // Allowed
let x = 3; // Not allowed
let x = 4; // Allowed
let x = 5; // Not allowed
Redeclaring a variable with let, in another scope, or in
another block, is allowed:
Example
let x = 2; // Allowed
let x = 3; // Allowed
let x = 4; // Allowed
Try it Yourself »
Hoisting
Variables defined with var are hoisted to the top. (Js
Hoisting)
You can use a variable before it is declared:
Example
// you CAN use carName here
var carName;
Try it Yourself »
Variables defined with let are not hoisted to the top.
Using a let variable before it is declared will result in a
ReferenceError.
The variable is in a "temporal dead zone" from the start
of the block until it is declared:
Example
// you can NOT use carName here
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