JavaScript Const keyword
The const keyword was introduced in
ES6 (2015).
Variables defined with const cannot be Redeclared.
Variables defined with const cannot be Reassigned.
Variables defined with const have Block Scope.
Cannot be Reassigned
A const variable cannot be reassigned:
Example
const PI = 3.141592653589793;
PI = 3.14; // This will give an error
PI = PI + 10; // This will also give an error
Try it Yourself »
Must be Assigned
JavaScript const variables must be assigned a value when they are declared:
Correct
const PI = 3.14159265359;
Incorrect
const PI;
PI = 3.14159265359;
When to use JavaScript const?
As a general rule, always declare a variable with const unless you know that the value will change.
Use const when you declare:
- A new Array
- A new Object
- A new Function
- A new RegExp
Constant Objects and Arrays
The keyword const is a little misleading.
It does not define a constant value. It defines a constant reference to a value.
Because of this you can NOT:
- Reassign a constant value
- Reassign a constant array
- Reassign a constant object
But you CAN:
- Change the elements of constant array
- Change the properties of constant object
Constant Arrays
You can change the elements of a constant array:
Example
// You can create a constant array:
const cars = ["Saab", "Volvo", "BMW"];
// You can change an element:
cars[0] = "Toyota";
// You can add an element:
cars.push("Audi");
Try it Yourself »
But you can NOT reassign the array:
Example
const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"]; // ERROR
Try it Yourself »
Constant Objects
You can change the properties of a constant object:
Example
// You can create a const object:
const car = {type:"Fiat", model:"500", color:"white"};
// You can change a property:
car.color = "red";
// You can add a property:
car.owner = "Johnson";
Try it Yourself »
But you can NOT reassign the object:
Example
const car = {type:"Fiat", model:"500", color:"white"};
car = {type:"Volvo", model:"EX60", color:"red"}; // ERROR
Try it Yourself »
Browser Support
The const keyword is not supported in Internet Explorer 10 or earlier.
The following table defines the first browser versions with full support for the const keyword:
| Chrome 49 | IE 11 / Edge | Firefox 36 | Safari 10 | Opera 36 |
| Mar, 2016 | Oct, 2013 | Feb, 2015 | Sep, 2016 | Mar, 2016 |
Block Scope
Declaring a variable with const is similar to let when it comes to Block Scope.
The x declared in the block, in this example, is not the same as the x declared outside the block:
You can learn more about block scope in the chapter JavaScript Scope.
Redeclaring
Redeclaring a JavaScript var variable is allowed anywhere in a program:
Example
var x = 2; // Allowed
var x = 3; // Allowed
x = 4; // Allowed
Redeclaring an existing var or let variable to const, in the same scope, is not allowed:
Example
var x = 2; // Allowed
const x = 2; // Not allowed
{
let x = 2; // Allowed
const x = 2; // Not allowed
}
{
const x = 2; // Allowed
const x = 2; // Not allowed
}
Reassigning an existing const variable, in the same scope, is not allowed:
Example
const x = 2; // Allowed
x = 2; // Not allowed
var x = 2; // Not allowed
let x = 2; // Not allowed
const x = 2; // Not allowed
{
const x = 2; // Allowed
x = 2;
// Not allowed
var x = 2;
// Not allowed
let x = 2;
// Not allowed
const x = 2; // Not allowed
}
Redeclaring a variable with const, in another scope, or in another block, is allowed:
Example
const x = 2; // Allowed
{
const x = 3; // Allowed
}
{
const x = 4; // Allowed
}
Const 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 const are also hoisted to the top, but not initialized.
Meaning: Using a const variable before it is declared will result in a ReferenceError:
