Javascript array in array - first index is always overwritten

Question

I have a problem with my JS function. For simplification, I want to fill an array (arr1) with n other arrays (arr2). In my loop I use a counter for the current postion in arr1 (cant use arr1.push for a reason). If I log all my arr2's in arr1 they are all the same, always the last one that was added. So I wrote a basic script to test it. I always log the first element and incement the counter. I'm new to JS, is there some huge misunderstanding I don't get?

function test(){
	var arr1 = [];
	var arr2 = [];
	var counter=1;
	arr2[0]="first";
	arr2[1]="first";
	arr2[2]="first";
	arr1[0]=arr2;
  arr1[0].forEach(function(elem){console.log(elem);});
	for (var i = 0; i < 10 ; i++) {
		arr2[0]=counter;
		arr2[1]=counter;
		arr2[2]=counter;
		arr1[counter]=arr2;
    
		arr1[0].forEach(function(elem){console.log(elem);});
		counter++;
	}
}

<button onclick="test()">Click</button>

... because arr1[0] = arr2; does not make a copy of arr2. — Pointy
– Pointy, Commented Oct 21, 2018 at 18:55

marc_s · Accepted Answer · 2019-01-20 17:27:10Z

1

You can try to use the spread operator.

arr1[0]=[...arr2];
arr1[counter]=[...arr2];

An array is a reference type, so you always refer to the base, you don't put a copy of it inside of arr1 but a reference to the arr2.

You want a copy of arr2 to be assigned to arr1.

You can do this by creating a new Array, or more modern the ... spread operator

edited Jan 20, 2019 at 17:27

marc_s

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answered Oct 22, 2018 at 0:19

Wolfyr

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Comments

Ikhlak S. · Accepted Answer · 2018-10-22 08:49:36Z

0

As Pointy said it, it just referenced the arr2 and doesn't create a copy.

So you need to

arr2=new Array();

at the beginning of the loop.

edited Oct 22, 2018 at 8:49

Ikhlak S.

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answered Oct 21, 2018 at 19:13

Henry

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