JavaScript - find index of array inside another array

Question

I am trying to find the index of an array inside another array in Javascript, as below:

const piece = [5, 10];
const array = [[5, 10], [5, 11]];

//Using indexOf
console.log(array.indexOf(piece));

//Using findIndex
console.log(array.findIndex(function(element) {
  return element == piece;
  }));

I'm expecting these to return a 0 as that is the index where my "piece" is inside the larger array, but both methods return -1

Any ideas why this is happening? And if there is a different way for me to do this?

Thanks.

They are different arrays that happen to have the same numbers, but still different arrays. — trincot, Jan 06 '22 at 17:58
Please follow the link to see a solution. https://stackoverflow.com/a/34170710/1635742 — Arul Prabakaran, Jan 06 '22 at 18:04

score 4 · Accepted Answer · edited Jan 06 '22 at 18:14

4

To compare the actual values you can use the JSON.stringify() method:

piece = [5, 10];
array = [[5, 10], [5, 11]];

//Using findIndex
console.log(array.findIndex(function(element) {
    return JSON.stringify(element) == JSON.stringify(piece);
    }));

edited Jan 06 '22 at 18:14

3limin4t0r

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answered Jan 06 '22 at 18:05

MWO

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score 1 · Answer 2 · answered Jan 06 '22 at 18:02

You are comparing objects (arrays) that were created each on their own, so they are not the same objects. If you want, you can compare each of the integers inside the arrays, and when also the array lengths match, you can conclude they are the "same":

piece = [5, 10];
array = [[5, 10], [5, 11]];

//Using findIndex
console.log(array.findIndex(function(element) {
  return element.length === array.length 
      && element.every((val, i) => val == piece[i]);
}));

score 0 · Answer 3 · answered Jan 06 '22 at 18:01

Arrays in JavaScript are not compared by value, but by reference. So if you have arrays a and b like this:

const a = [1, 2];
const b = [1, 2];

When you compare them using ==, or === then their references are compared, not values. So this console.log(a == b), or console.log(a === b) will print false.

JavaScript - find index of array inside another array

3 Answers3