Returning on conditional if from inside Array.filter

Question

I'm trying to return the index of an item that meets a certain condition from within an Array.filter method, but I still keep getting an array of actual values instead of the indexes of the values I want.

Example:

var seq = [3, 4, 1, 2, 1];

seq.filter((curr, index) => {
  if (seq[index + 1] < seq[index]) {
    return index;
  }
});

// returns [4, 2]
// want to return [1, 3] (the indexes of 4 & 2)

Is there a more efficient method I can use? I'm trying to avoid using a for loop. Thanks.

Answer 1

You can use reduce here

 var seq = [3, 4, 1, 2, 1]; var result = seq.reduce((ar, curr, index) => { if (seq[index + 1] < seq[index]) ar.push(index); return ar; }, []); console.log(result) 

Answer 2

You can use filter with an array of the indices and the right test for the return

 var seq = [3, 4, 1, 2, 1], result = seq.map(function (_, i) { return i; }).filter(function (i) { return seq[i - 1] < seq[i]; }); console.log(result); 

The better solution would be a loop with forEach

 var seq = [3, 4, 1, 2, 1], result = []; seq.forEach(function (a, i) { seq[i - 1] < a && this.push(i); }, result); console.log(result); 

Answer 3

It's not a bad idea to use a for loop in this case. I personally think it's cleaner and more efficient.

let seq = [3, 4, 1, 2, 1];
for (let i = 0; i < seq.length; i++) {
  if (seq[i + 1] < seq[i]) {
    continue;
  }
  seq.splice(i, 1);
}