Slide 1

When to use JavaScript const?

As a general rule, always declare a variables with const unless you know that the value will change.

Always use const when you declare:

• A new Array
• A new Object
• A new Function
• A new RegExp

Slide 2

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 a constant array
• Change a constant object

Slide 3

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");

Slide 3 DownWards

But you can NOT reassign the array:

Example

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

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

Slide 4

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";

But you can NOT reassign the object:

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

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