Typescript/Javascript array push pushes to all arrays in 2d array

Question

This should be very basic but do not working. I am trying to push an element on 2d number array on a specific index, but it is inserting element on every index.

let adj = new Array(4).fill(new Array())

adj[0].push(1)
console.log(adj)

The output I am getting:

[[1], [1], [1], [1]]

But I am expecting:

[[1], [], [], []]

@NajamUsSaqib why? just why? yes it gets to the right result but still, why? — Thomas
– Thomas, Commented Aug 23, 2022 at 12:02
@Amit, how big is your array? is there a reason you use new Array(4).fill(new Array()) instead of [[], [], [], []]? — Thomas
– Thomas, Commented Aug 23, 2022 at 12:04
@Thomas developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Najam Us Saqib
– Najam Us Saqib, Commented Aug 23, 2022 at 12:04
@Thomas I want a dynamic 2d array where length is fixed for main array. But all internal array size is dynamic. If I be very specific, I want to create a adjacency list for a graph where each array index means the node number and each index(node) will contant the node which has path/connection to it. — Amit Biswas
– Amit Biswas, Commented Aug 23, 2022 at 12:09

Marko Borković · Accepted Answer · 2022-08-23 11:56:26Z

4

What you're doing with let adj = new Array(4).fill(new Array()) is just setting adj to an array of references to the same array, thus changing one, changes the others.

To fix this do it like this

let adj = Array.from(Array(4), () => new Array());
adj[0].push(1);
console.log(adj)

answered Aug 23, 2022 at 11:56

Marko Borković

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3 Comments

Amit Biswas Over a year ago

@Marco Thanks for your workable code. But I have one more confusion then. Why I need Array.from where this is already an array.

Marko Borković Over a year ago

Because if you try to fill the already existing array it will be filled with references to the same object. When using Array.from the arrow function argument () => new Array() is called for every element, thus creating a list of unique arrays. In other words creating a 2D array.

Amit Biswas Over a year ago

@Marco ok from your idea, finally I have got another two ways. let adj = new Array(4).fill(null).map(() => []) let adj = [...new Array(4)].map(() => [])

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