foreach not updating element in array

Question

I want to update a property in each element in my array to be a particular index. However when I try the following:

  static reindexComponentsOnMultiplePages(components) {
    return components.forEach((el, idx) => (el.componentIndex = idx));
  }

I get returned undefined.

My current array is as follows:

components = [ 
               [ {...}, {...}, {...} ], 
               [ {...}, {...}, {...} ] 
             ]

I was expecting each property in each element in my array to be updated. I call the method as follows:

pageComponents = MyService.reindexComponentsOnMultiplePages(pageComponents);

Yes, that's expected, the return value of forEach is undefined. What did you expect to get? — Teemu
– Teemu, Commented Mar 4, 2020 at 8:45

vicpermir · Accepted Answer · 2020-03-04 08:56:34Z

3

Array.forEach() returns undefined as described in the documentation.

There's no need to return the array, you are updating the items in your forEach loop.

var arr = [{name: 'A', componentIndex: null}, {name: 'B', componentIndex: null}];
console.log(arr);

arr.forEach(function(item, index) {
  item.componentIndex = index;
});

console.log(arr);

Just avoid returning anything, you don't need to.

answered Mar 4, 2020 at 8:56

vicpermir

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2 Comments

methuselah Over a year ago

Hi my array structure is as follows components = [ [ {...}, {...}, {...} ], [ {...}, {...}, {...} ] ] - basically an array of objects within an array of arrays.

vicpermir Over a year ago

And what is componentIndex to you? Imagine [[A, B], [C, D]] do you want you indices to be [[0, 1], [0, 1]] or [[0, 1], [2, 3]] ?

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