push value in map of arrays at a specific postition

Question

I am creating a map of array using this:

var m = new Map(Array(40).fill(new Array()).entries());

Now, I want to push values in those array. But when I do this:

m.get(1).push(10)

The value 10 gets pushed into all the arrays instead of the one at 1st position.

Suren Srapyan · Answer 1 · 2017-10-09T09:45:00.607

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fill gets single array an uses it to fill all rows of the given array, it doesn't create a new array for each row. This means that your single array reference is shared between all rows. Because array is a reference type, you use the single reference to manipulate it, so the actual object is changed. You can check this by comparing the references of each row.

const arr = new Array(2).fill(new Array());

console.log(arr[0] === arr[1]);

For creating separate arrays, you can see @Nina's answer above

edited Oct 09 '17 at 09:45

answered Oct 09 '17 at 08:20

Suren Srapyan

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You're only confirming what the OP figured out... – Cerbrus Oct 09 '17 at 08:30
Need each answer to have full answer to the question, every aspect??? – Suren Srapyan Oct 09 '17 at 08:31
Can't it refer some part to another one? – Suren Srapyan Oct 09 '17 at 08:32
This doesn't answer the question. You're not explaining how to properly set a value in only one position of the "map". – Cerbrus Oct 09 '17 at 08:33
See also the above comment – Suren Srapyan Oct 09 '17 at 08:34
_"Look at this other answer"_ ___Isn't an answer___. If I realize changing my car's wheels in a swamp isn't working, and want to know how to change a wheel, _"Yea, cars tend to sink in swamps"_ doesn't answer the question. – Cerbrus Oct 09 '17 at 08:34
Can't be it be like to a `link to another source`. What if the source is another answer? – Suren Srapyan Oct 09 '17 at 08:36
A answer can't be a _"link to another source"_. If that other source changes or is removed, the answer loses all value. Come on, with 20k, you should know this by now. – Cerbrus Oct 09 '17 at 08:38

score 1 · Accepted Answer · answered Oct 09 '17 at 08:22

1

You could take another pattern to build independent arrays.

var m = new Map(Array.from({ length: 40 }, _=> []).entries());
m.get(1).push(10);

console.log([...m]);

answered Oct 09 '17 at 08:22

Nina Scholz

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push value in map of arrays at a specific postition

2 Answers2