Pushing array into array then handling elements inside

Question

so I just came across this, but could anyone explain it to me, why is this phenomenon present here? I didn't find any help online...

1.So if I were to get the following two arrays:

let a = [];
let b = ['','',''];

2.and then push array b into array a three times:

a.push(b);
a.push(b);
a.push(b);

3.but then when I try to change the first element of the first array like so:

a[0][0] = 'foo';

4.then all the first element of ALL the arrays change, not just the first one, as console.log(a) outputs:

(3) [Array(3), Array(3), Array(3)]
0: Array(3)
  0: "foo"
  1: ""
  2: ""

1: Array(3)
  0: "foo"
  1: ""
  2: ""

2: Array(3)
  0: "foo"
  1: ""
  2: ""

I would like to ask, why does this happen and in what way could I change an element based on its index inside of array a

The indices in a are all holding a reference to b -- "they all get updated" is a bit misleading, you're updating one object and it appears in multiple places. — skyline3000
– skyline3000, Commented Dec 6, 2020 at 4:53

JBallin · Accepted Answer · 2020-12-06 04:54:31Z

2

b is a pointer to a mutable array, changing the array it points to will cause the other places you point to it to update as well.

You would either push copies of b into the array or create a modified copy of b when trying to mutate the value it points to.

answered Dec 6, 2020 at 4:54

JBallin

9,9654 gold badges52 silver badges63 bronze badges

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

Comments

tom · Accepted Answer · 2020-12-06 04:53:25Z

1

Because you're passing b which is just a reference to an object. To have the behavior you want, change your code to be like this:

a.push(b.map((x) => x))

For more see this page.

answered Dec 6, 2020 at 4:53

tom

1,5031 gold badge18 silver badges31 bronze badges

Comments

MarketMan39 · Accepted Answer · 2020-12-06 04:54:04Z

1

if you console.log(b), you'll see that as well.
Try doing b[1] = "test" then check the value of a and you'll see all the b's that were pushed into there have test in them too.
b is a reference so anytime you change the value of it, you are altering the places it is referenced to.

answered Dec 6, 2020 at 4:54

MarketMan39

2182 silver badges9 bronze badges

Collectives™ on Stack Overflow

Pushing array into array then handling elements inside

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related