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ECMAScript 2015
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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
// code here can use carName
function myFunction() {
// code here can also use carName
}
Global variables can be accessed from anywhere in a JavaScript program.
Function Scope
Variables declared Locally (inside a function) have Function Scope.
Example
function myFunction() {
var carName = 'Volvo';
// code here CAN use carName
}
// code here can NOT use carName
Local variables can only be accessed from inside the function where they are declared.
JavaScript Block Scope
Variables declared with the var keyword cannot have Block Scope.
Variables declared inside a block {} can be accessed from outside the block.
Example
Before ES2015 JavaScript did not have Block Scope.
Variables declared with the let keyword can have Block Scope.
Variables declared inside a block {} cannot be accessed from outside the block:
Example
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:
Example
// 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:
Example
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10
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 | 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
for (var i = 0; i < 10; i++) {
// some statements
}
// Here i is 10
Using let in a loop:
Example
for (let i = 0; i < 10; i++) {
// some statements
}
// Here i is 5
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.
Function Scope
Variables declared with var and let are quite similar when declared inside a function.
They will both have Function Scope:
var carName = 'Volvo'; // Function Scope
}
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:
Global 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
// code here can use window.carName
Global variables defined with the let keyword do not belong to the window object:
Example
// code here cannot use window.carName
Redeclaring
Redeclaring a JavaScript variable with var is allowed anywhere in a program:
Example
Try it Yourself »Redeclaring a var variable with let, in the same scope, or in the same block, is not allowed:
Example
let x = 3; // Not allowed
{
var x = 4; // Allowed
let x = 5 // Not allowed
}
Redeclaring a let variable with let, in the same scope, or in the same block, is not allowed:
Example
let x = 3; // Not allowed
{
let x = 4; // Allowed
let x = 5; // Not allowed
}
Redeclaring a let variable with var, in the same scope, or in the same block, is not allowed:
Example
var x = 3; // Not allowed
{
let x = 4; // Allowed
var x = 5; // Not allowed
}
Let 24 Assignments Questions
Redeclaring a variable with let, in another scope, or in another block, is allowed:
Example
Let 24 Assignments Online
{
let x = 3; // Allowed
}
{
let x = 4; // Allowed
}
Hoisting
Variables defined with var are hoisted to the top and can be initialized at any time (if you don't know what Hoisting is, read our Hoisting Chapter).
Meaning: You can use the variable before it is declared:
Example
This is OK:
Try it Yourself »Variables defined with let are hoisted to the top of the block, but not initialized.
Meaning: The block of code is aware of the variable, but it cannot be used until it has been declared.
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
This will result in a ReferenceError: