Add values to mapped array

Question

I have an array with this values:

0:
code: "fb"
description: "Not relevant text."
did: 1
name: "Some random name"
sortOrder: 1

1:
code: "fr"
description: "Not relevant text."
did: 2
name: "Some random name"
sortOrder: 2

When i map the array like this:

  values: this.stackOverflowExample.map(v => v.code).push(null)

And push a null code to the set, it sets all values to null

The output I want:

values: [
0: 'fb'
1: 'fr'
2: null ]

The output I got:

values: 3

How do I add values to the array this.stackOverflowExample which is mapped by code, without affecting the other values?

The Array.prototype.push method doesn't return the array it is called on. Rather, it returns the length of the array after adding the element. Either use a temporary variable, as in const a = this.stackOverflowExample.map(v => v.code).push(null); values: a` or to add the element using a method that returns the array such, i.e. Array.prototype.concat, as in values: this.stackOverflowExample.map(v => v.code).concat([null]) — Aluan Haddad
– Aluan Haddad, Commented Jun 2, 2021 at 12:19
@AluanHaddad Thank you for your answear with the concat method for the array , it does exactly what I wanted to achive. — Filip Huhta
– Filip Huhta, Commented Jun 2, 2021 at 12:38

Wing · Accepted Answer · 2021-06-02 12:40:35Z

8

The push method returns the new length of the array and not the mutated array, see documentation.

If you just want to append null to the mapped array you could either:

Concat your mapped array with `[null]`

values: anArray.map(mapFn).concat([null])

Spread your mapped array in an array with the last item being `null`

values: [...anArray.map(mapFn), null]

edited Jun 2, 2021 at 12:40

answered Jun 2, 2021 at 12:13

Wing

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2 Comments

Filip Huhta Over a year ago

Actualy this example returns: 0: (2) ['fb', 'fr'] 1: null Which is not exactly what i wanted to achive. The this.stackOverflowExample.map(v => v.code).concat([null]) from the comment returned the wanted output: 0: 'fb' 1: 'fr' 2: null But thanks anyway, for the explanation of the push method :)

Wing Over a year ago

You may have forgotten to spread your mapped array. .concat([null]) works too. I will update my answer to clarify this and add the other option.

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Add values to mapped array

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Concat your mapped array with `[null]`

Spread your mapped array in an array with the last item being `null`

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Concat your mapped array with [null]

Spread your mapped array in an array with the last item being null

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Concat your mapped array with `[null]`

Spread your mapped array in an array with the last item being `null`