JavaScript Let
The let keyword was introduced in
ES6 (2015)
Variables declared with let have Block Scope
Variables declared with let must be Declared before use
Variables declared with let cannot be Redeclared in the same scope
Block Scope
Before ES6 (2015), JavaScript did not have Block Scope.
JavaScript had Global Scope and Function Scope.
ES6 introduced the two new JavaScript keywords: let and const.
These two keywords provided Block Scope in JavaScript:
Example
Variables declared inside a { } block cannot be accessed from outside the block:
{
let x = 2;
}
// x can NOT be used here
Global Scope
Variables declared with the var always have Global Scope.
Variables declared with the var keyword can NOT have block scope:
Example
Variables declared with varinside a { } block can be accessed from
outside the block:
{
var x = 2;
}
// x CAN be used here
Cannot be Redeclared
Variables defined with let can not be redeclared.
You can not accidentally redeclare a variable declared with let.
With let you can not do this:
let x = "John Doe";
let x = 0;
Variables defined with var can be redeclared.
With var you can do this:
var x = "John Doe";
var x = 0;
Redeclaring 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:
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:
Difference Between var, let and const
| Scope | Redeclare | Reassign | Hoisted | Binds this | |
| var | No | Yes | Yes | Yes | Yes |
| let | Yes | No | Yes | No | No |
| const | Yes | No | No | No | No |
What is Good?
let and const
have block scope.
let and const
can not be redeclared.
let and const
must be declared before use.
let and const
does not bind to this.
let and const
are not hoisted.
What is Not Good?
var does not have to be declared.
var is hoisted.
var binds to this.
Browser Support
The let and const keywords are
not supported in Internet Explorer 11 or earlier.
The following table defines the first browser versions with full support:
| Chrome 49 | Edge 12 | Firefox 36 | Safari 11 | Opera 36 |
| Mar, 2016 | Jul, 2015 | Jan, 2015 | Sep, 2017 | Mar, 2016 |
Redeclaring
Redeclaring a JavaScript variable with var is allowed
anywhere in a program:
With let, redeclaring a variable in the same block is NOT allowed:
Example
var x = 2; // Allowed
let x = 3; // Not allowed
{
let x = 2; // Allowed
let x = 3; // Not allowed
}
{
let x = 2; // Allowed
var x = 3; // Not allowed
}
Redeclaring a variable with let, in another block, IS allowed:
Let Hoisting
Variables defined with var are hoisted to the top
and can be initialized at any time.
Meaning: You can use the variable before it is declared:
If you want to learn more about hoisting, study the chapter JavaScript Hoisting.
Variables defined with let are also hoisted to the top
of the block, but not initialized.
Meaning: Using a let variable before it is declared will result in a
ReferenceError:
Video: JavaScript let
