Using concat on non-existing index of array

Question

So, code below throws undefined if index is not existing in the array

Note: it's array of arrays

arr[index] = arr[index].concat(data);

Eventually I've fixed it with check

if (!array[index]) {
   array[index]=[];
}

Which solved the problem.

Question: is there any JavaScript trick which could solve this problem easier (less lines of code, without checks, whatever)?

Both of the proposed answers still involve a check like your original if statement, they just present it in a different way. — Jason Sperske
– Jason Sperske, Commented Sep 22, 2015 at 17:12

Buzinas · Accepted Answer · 2015-09-22 17:10:54Z

1

You can use the || oprator:

arr[index] = (arr[index] || []).concat(data);

It will check if the arr[index] is a truthy value (not null, undefined, NaN, "" and 0).

If it is, it will use it, and if it's not, it will use the [] to concat to data.

edited Sep 22, 2015 at 17:10

answered Sep 22, 2015 at 17:06

Buzinas

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score 1 · Accepted Answer · 2015-09-22 18:31:06Z

1

In ES6:

((a = []) => a.concat(data))(array[index])

but this boils down to mere code golf, although it does have the advantage that it's a few characters shorter, and the [] default will be used only if array[index] is missing (undefined).

edited Sep 22, 2015 at 18:31

answered Sep 22, 2015 at 17:15

user663031

Comments

Vinay · Accepted Answer · 2015-09-22 17:06:30Z

0

Not without compromising readability:

array[index] = (array[index] ? array[index] : []).concat(data);

Note that this and the method of using the || operator:

array[index] = (array[index] || []).concat(data);

Are both technically unsafe since they check if array[index] is truthy.

answered Sep 22, 2015 at 17:06

Vinay

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