How do variable assignment with arrays behave in javascript?

Question

I'm trying to understand the exact behavior of arrays in Javascript. I know that they work by reference. But I was confused when I came across this piece of code:

let arr1 = [1,2,3];

let arr2 = arr1;

arr1[2] = 4;

console.log(arr2); // [1,2,4]

arr1 = [2, 3, 4];

console.log(arr2); // [1,2,4] why not [2, 3, 4]

So how exactly do arrays behave in javascript?

That’s not a question about arrays, but about variables. When you write arr1 = [2, 3, 4];, a new array [2, 3, 4] is created and assigned to arr1. That new array has nothing to do with arr2 or the other array anymore. — Sebastian Simon
– Sebastian Simon, Commented Aug 4, 2018 at 23:58

charlietfl · Accepted Answer · 2018-08-04 23:58:52Z

2

When you do let arr2 = arr1; both variables share reference to same actual array.

However if you reassign one of those variables to a different array they no longer share same reference and are completely isolated from each other

answered Aug 4, 2018 at 23:58

charlietfl

172k14 gold badges125 silver badges154 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

sv123 Over a year ago

Is this due to immutability nature?

charlietfl Over a year ago

No...has nothing to do with immutability. Simply a value change. The new value points to a completely different object(array)

Andrew Li Over a year ago

@sv123 No, arrays are mutable. It's just the fact you're completely reassigning to a whole new array and thus a whole new reference (to array in memory)

alariva · Accepted Answer · 2018-08-05 02:33:50Z

2

Think of it like this, a variable is a pointer for an object in memory, now:

Line 1: Assingned pointer (arr1) to object [1,2,3]
Line 2: Assigned a pointer for a pointer which points to object [1,2,3], this is what is meant by reference.
Line 3: changed object [1,2,3]'s last value to 4. At this point arr1 and arr2 still points to this object.
Line 4: Assinged the pointer arr1 a NEW object with content [2,3,4]. At this point arr2 points still to the old [1,2,4] object.

edited Aug 5, 2018 at 2:33

alariva

2,1471 gold badge25 silver badges38 bronze badges

answered Aug 5, 2018 at 0:01

ibrahim tanyalcin

6,5634 gold badges21 silver badges27 bronze badges

Collectives™ on Stack Overflow

How do variable assignment with arrays behave in javascript?

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related