Filtering an array while slicing another array in the filter callback function in Javascript

Question

I am trying to filter an array A while slicing an array B using the index in the callback function filter.

const endResult = answers[user].filter((ans, index) => {

    if (ans !== '') {
        return true
    } else {
        console.log(index)
        outsideArray.splice(index, 1)
        return false
    }
})

It seems I can't use the index inside the filter callback function to splice another array. Is there a way to do this? I tried various things like map() without much success.

outsideArray: Array [ 1, 2, 4, 5 ]

answer[users]: Array [ "fff", "", "", "fffffff", "", "" ]

In the end, I get:

outsideArray: Array [ 1, 4 ]

answer[users]: Array [ "fff", "fffffff" ]

I should get:

outsideArray: Array [ ]

answer[users]: Array [ "fff", "fffffff" ]

add in your input data and expected output so that we know what you are trying to do — Prasanna
– Prasanna, Commented Mar 27, 2018 at 18:13
what do you expect your outsideArray to look after the operation has finished ?? — Prasanna
– Prasanna, Commented Mar 27, 2018 at 18:16

ethan.roday · Accepted Answer · 2018-03-27 18:37:37Z

1

You don't actually want to splice the exactly corresponding indices in your outsideArray; you want to remove values that match certain indices in answer[users]. Try this:

outsideArray.splice(outsideArray.indexOf(index), 1)

For example, in the second iteration of the filter call, we have the following:

index is 1
outsideArray is [ 1, 2, 4, 5 ]

Now, if you splice outsideArray at index 1, you'll get [ 1, 4, 5 ]. This is not that you want. You want to splice outsideArray at the index whose value is 1. That's where indexOf comes in:

outsideArray.indexOf(index) returns the first index in outsideArray where the value is equal to 1. This is at index 0.

If we splice outsideArray at index 0, we get what we want: [ 2, 4, 5 ].

Depending on your use case, there are definitely more efficient ways to design this. Currently, you're searching the entirety of outsideArray in every iteration of the filter call.

edited Mar 27, 2018 at 18:37

answered Mar 27, 2018 at 18:20

ethan.roday

2,6852 gold badges27 silver badges29 bronze badges

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

3 Comments

henry Over a year ago

Ok my bad, I was deleting the elements found in the index instead of the values themselves!

ethan.roday Over a year ago

Right. Although, as noted in my edit. there are probably more efficient ways to go about this depending on your use case.

Eric Ferreira Over a year ago

outsideArray.findIndex(val => val === index) unnecessarily creates a predicate to test simple equality. It's preferable to use indexOf when testing simple equality.

Prasanna · Accepted Answer · 2018-03-27 18:28:17Z

1

if you want to delete your array element in increasing order of index, you have to remove the first element each time you get an empty string.

const endResult = answers[user].filter((ans, index) => {
  if (ans !== '') {
      return true
  } else {
      outsideArray.splice(0, 1) // or outsideArray.shift()
      return false
  }
})

If in case you want your indexes to control what to delete from the array you would have to check for your callback's index and array's index match

const endResult = answers[user].filter((ans, index) => {
  const outsideArrayIndex = outsideArray.indexOf(index);
  if (outsideArrayIndex > -1) {
    outsideArray.splice(outsideArrayIndex,1);
    return false;
  }
  return true;
})

answered Mar 27, 2018 at 18:28

Prasanna

4,7461 gold badge26 silver badges47 bronze badges

Comments

Eric Ferreira · Accepted Answer · 2018-03-27 18:28:45Z

When you use the index directly to remove elements from the outside array, you are referring to the index in the outside array, not the value of each element, which is what it seems you are wanting.

outsideArray: Array [ 1, 2, 4, 5 ]
        (Indices are: 0  1  2  3)

So when you remove at index 1, you remove the item with value 2. The array is now:

outsideArray: Array [ 1, 4, 5 ]
        (Indices are: 0  1  2)

When you remove at index 2, you remove the item with value 5. The new array is:

outsideArray: Array [ 1, 4 ]
        (Indices are: 0  1)

Now you remove at indices 4 and 5, but those items don't exists so it doesn't do anything.

What you seem to actually want is to remove the items by value. You can do this by searching the array for the item that matches the value of the index, like so:

outsideArray.splice(outsideArray.indexOf(index), 1)

Collectives™ on Stack Overflow

Filtering an array while slicing another array in the filter callback function in Javascript

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related