JavaScript Const Keyword

Cannot be Reassigned

A variable defined with the const keyword cannot be reassigned:

const PI = 3.141592653589793;
PI = 3.14;      // This will give an error
PI = PI + 10;   // This will also give an error

Must be Assigned

JavaScript const variables must be assigned a value when they are declared:

const PI = 3.14159265359;

const PI;
PI = 3.14159265359;

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:

// 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");

But you can NOT reassign the array:

const cars = ["Saab", "Volvo", "BMW"];

cars = ["Toyota", "Volvo", "Audi"];    // ERROR

Constant Objects

You can change the properties of a constant object:

// 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";

But you can NOT reassign the object:

const car = {type:"Fiat", model:"500", color:"white"};

car = {type:"Volvo", model:"EX60", color:"red"};    // ERROR

Difference Between var, let and const

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:

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:

const x = 10;
// Here x is 10

{
const x = 2;
// Here x is 2
}

// Here x is 10

You can learn more about block scope in the chapter JavaScript Scope.

Redeclaring

Redeclaring a JavaScript var variable is allowed anywhere in a program:

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:

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:

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:

const x = 2;       // Allowed

{
  const x = 3;   // Allowed
}

{
  const x = 4;   // Allowed
}

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:

carName = "Volvo";
var carName;

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:

alert (carName);
const carName = "Volvo";