Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

JavaScript JavaScript Arrays Store Multiple Values in an Array Copy and Combine Arrays with the Spread Operator

Posted by trottly
trottly
5,394 Points

Modifying array that was declared with `const` keyword?

How are we able to modify an array (for example, using the array methods to add or remove elements) that has been declared with the const keyword? Wouldn't this make the array immutable to begin with?

2 Answers

Cameron Childres
Cameron Childres
11,820 Points
Cameron Childres
Cameron Childres
11,820 Points

Hi Trottly,

The keyword const can be a little misleading. From w3schools.com page on const:

It does NOT define a constant value. It defines a constant reference to a value.
Because of this, we cannot change constant primitive values, but we can change the properties of constant objects.

This means that we can change, add, or remove items within the referenced array -- but we can't change the overall reference.

This is possible:

const numbers = [1, 2, 3]
numbers[0] = 0 
// array is now [0, 2, 3]

But this is not:

const numbers = [1, 2, 3]
numbers = [0, 2, 3]
// produces error

The first example changes the value of an item in the array, where the second example attempts to reassign the variable to a new array. I hope this helps! Let me know if you have any questions.

trottly
trottly
5,394 Points

Is there a way to make an array thats elements may also not be changed?

Cameron Childres
Cameron Childres
11,820 Points
Cameron Childres
Cameron Childres
11,820 Points

You can with Object.freeze():

const numbers = [1, 2, 3];
Object.freeze(numbers);
numbers[0] = 0;
console.log(numbers); 
// prints [1, 2, 3]
Busra Tokuc
Busra Tokuc
7,444 Points
Busra Tokuc
Busra Tokuc
7,444 Points

Hi, You can check this article.