Pushing an array to another array

Question

I'm new to javascript and I've been going through some exercises but I can't figure out why this code works like this,

    let newArray = [];
    let arr1 = [];
    for (let i = 0; i < 2; i++) {

      for (let j = 0; j < 2; j++) {

        arr1.push(0);
        //console.log('Arr1:',arr1); 
      }

      newArray.push(arr1);
      //console.log('New Array:', newArray)
      
    }
    console.log(newArray);

According to me, this block of code should output [[0,0][0,0,0,0]] but the real output is [[0,0,0,0][0,0,0,0]].

I tried console logging after every iteration(by removing the comments) and I can't seem to find where it goes wrong. It seems like it all goes well until the last log. I'd appreciate your help.

newArray contains arr1 twice. After the first newArray.push(arr1), the value that was just added to newArray is a reference to arr1 not the values of arr1. After that, when you modify arr1, the first value in newArray is also modified. — Titus
– Titus, Commented Oct 13, 2021 at 0:05
Oh, so even if I've already pushed arr1 once, if I modify it after that, the pushed arr1 is modified too? — g97
– g97, Commented Oct 13, 2021 at 0:08
Yes, that is correct, you can clone the array if you want to get the result that you've mentioned, something like newArray.push([...arr1]). [...arr1] will create a new array that contains all the value of arr1 at that point. — Titus
– Titus, Commented Oct 13, 2021 at 0:11
Thanks but what about this code: (I dont know how to format multiple lines as code so I will post all code in next comment) — g97
– g97, Commented Oct 13, 2021 at 0:23
let newArray = []; let arr1 = [1,2,3,4,5,6]; newArray.push(arr1); console.log(newArray); arr1=[7,8,9,10]; newArray.push(arr1) console.log(newArray); — g97
– g97, Commented Oct 13, 2021 at 0:23

ad007 · Accepted Answer · 2021-10-13 01:33:15Z

1

One way to do it is to avoid using push() function and use concat() function instead. More about it on existing discussion.

You would have something like:

newArray = newArray.concat(arr)

answered Oct 13, 2021 at 1:33

ad007

1464 bronze badges

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

1 Comment

g97 Over a year ago

Thanks but I was just curious about how that piece of code worked like that. Wasn't trying to copy arrays.

Unmitigated · Accepted Answer · 2021-10-13 01:34:57Z

1

You should use Array#slice or spread syntax to copy the array instead of pushing the same reference multiple times, since subsequent changes will continue to be reflected.

let newArray = [];
let arr1 = [];
for (let i = 0; i < 2; i++) {
  for (let j = 0; j < 2; j++) {
    arr1.push(0);
  }
  newArray.push(arr1.slice());
}
console.log(JSON.stringify(newArray));

answered Oct 13, 2021 at 1:34

Unmitigated

91.4k12 gold badges102 silver badges109 bronze badges

3 Comments

g97 Over a year ago

Thanks but I was just curious about how that piece of code worked like that. Wasn't trying to copy arrays.

Unmitigated Over a year ago

It works like that because you pushed the same array twice, then added more elements to that array. Changes to that array are reflected in both references to that array, so it displays two arrays with 4 elements (actually the same array with 4 elements twice).

g97 Over a year ago

I still haven't learned about oop or the differences of references and values. I was getting ahead of myself. Just couldn't get my head around it. Thanks though.

Collectives™ on Stack Overflow

Pushing an array to another array

2 Answers 2

1 Comment

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related