JavaScript array - splice call within forEach unexpected result

Question

I noticed a bug within my code and tried to reproduce it with simpler JavaScript shown below. The goal is to sort an array and compare element 'x' to 'y' but not waste time comparing 'y' to 'x'. Note that I am also using objects comparing properties in my other code.

var a = [1, 3, 2, 5];
a.sort((b, c) => b - c);
a.slice().forEach((x, ind) => {
  console.log(a.splice(ind, 1));
  a.forEach(z => { console.log(z); });
});

The remaining elements of the above are 2 and 5. I would expect that elements 3 and 5 are removed. (Works the same with or without the copy using slice())

I ended up just reverting to for loops using indexes where the inner array's index starts at the outer array's index + 1.

You are iterating through an array and in between deleting an item of it, which will rearrange the index and this will change the behavior of loop in the following iteration. — Satish Kumar
– Satish Kumar, Commented Mar 30, 2018 at 22:18

dave · Accepted Answer · 2018-03-30 22:10:57Z

3

Well, you start with

[1, 2, 3, 5]

and you remove the element at the 0 index, ending up with:

[2, 3, 5]

In your next iteration, you remove the element at the 1 index, ending up with

[2, 5]

In your next iteration, you remove the element at the 2 index, which doesn't exist, so you still have

[2, 5]

You probably want to do:

console.log(a.splice(ind, 0));

answered Mar 30, 2018 at 22:10

dave

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Comments

BShaps · Accepted Answer · 2018-03-30 22:12:47Z

0

I believe you are asking why you are not getting the results you want from that example. The reason behind this is that you are altering the length of array a in your loop while expecting the indices to not change.

So by the time you reach ind = 2, you are trying to remove items from an index that doesn't exist. To fix this you would need to keep a count of how many items you have removed and change console.log(a.splice(ind, 1)); to be console.log(a.splice(ind - itemCount, 1));

answered Mar 30, 2018 at 22:12

BShaps

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