array in callback for reduce is not an array?

Question

I have a callback that I am using like

return innerArray.reduce((prev, curr, i, array) => {

However, when I try to use the array (for me this is an array of objects) like

array[i-1].outerIndex

I get a JavaScript error that I cannot read a property of undefined, reading outerIndex. The only way I was able to get this to work is with

let prevObj = { ...array[i - 1] };
prevObj.outerIndex

Is there something that is not an array in the argument passed to the reduce callback?

Quentin · Accepted Answer · 2022-01-20 14:14:05Z

1

Is there something that is not an array in the argument passed to the reduce callback?

No. It is an array.

The error message says that array[i-1] is undefined.

This is probably when it is dealing with the first item in the array so i is 0 so you are trying to access array[-1] which is out of bounds.

answered Jan 20, 2022 at 14:14

Quentin

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