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 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;

When to use JavaScript const?

Always declare a variable with const when you know that the value should not be changed.

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:

// 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:

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