Why are items inside an Array() dependent? (Javascript)

Question

I did:

a = Array(2).fill([]);
a[0].push(1);

Expected outcome: a = [[1], []] Got: a = [[1], [1]]

Why does the second item depend on the first one here?

Thanks in advance.

Pointy · Accepted Answer · 2020-08-23 14:42:38Z

3

The call to .fill() passes a single empty array. Thus, that same array is dropped into all the elements of the filled array.

Consider that if you called Array(2).fill(3); all the elements would contain the value 3. It's really no different here.

answered Aug 23, 2020 at 14:42

Pointy

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Comments

User863 · Accepted Answer · 2020-08-23 14:46:40Z

1

From docs,

If the first parameter is an object, each slot in the array will reference that object.

Alternate using Array.from()

const a = Array.from({length: 2}, i => [])

a[0].push(1)

console.log(a)

answered Aug 23, 2020 at 14:46

User863

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