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The let keyword was introduced inES6 (2015)
Variables defined with let cannot be redeclared.
You cannot accidentally redeclare a variable.
With let you can not do this:
let x = "John Doe";
let x = 0;
// SyntaxError: 'x' has already been declared
With var you can:
var x = "John Doe";
var x = 0;
Before ES6 (2015), JavaScript had only Global Scope and Function Scope.
ES6 introduced two important new JavaScript keywords: let and const.
These two keywords provide Block Scope in JavaScript.
Variables declared inside a { } block cannot be accessed from outside the block:
{
let x = 2;
}
// x can NOT be used here
Variables declared with the var keyword can NOT have block scope.
Variables declared inside a { } block can be accessed from outside the block.
{
var x = 2;
}
// x CAN be used here
Redeclaring a variable using the var keyword can impose problems.
Redeclaring a variable inside a block will also redeclare the variable outside the block:
var x = 10;
// Here x is 10
{
var x = 2;
// Here x is 2
}
// Here x is 2
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:
let x = 10;
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10
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 | Edge 12 | Firefox 44 | Safari 11 | Opera 36 |
| Mar, 2016 | Jul, 2015 | Jan, 2015 | Sep, 2017 | Mar, 2016 |
Redeclaring a JavaScript variable with var is allowed
anywhere in a program:
var x = 2;
// Now x is 2
var x = 3;
// Now x is 3
With let, redeclaring a variable in the same block is NOT allowed:
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 x = 2; // Allowed
{
let x = 3; // Allowed
}
{
let x = 4; // Allowed
}
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:
This is OK:
carName = "Volvo";
var carName;
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:
carName = "Saab";
let carName = "Volvo";