How to push new elements to an array with undefined index in JavaScript

Question

I want to create array once and then just push values to it with any index , but i get Cannot read property 'push' of undefined error

I have following scenario

var neg = [];

I want to use push randomly with any index

neg[0].push([1,2]);

or

neg[22].push([1,2]);

right now I want to define manually like neg[0] = []; , Is there any one way where i can just push to any index i want ?

`neg[0]` is not an array, so your trying to effectively push something onto an undefined value — Derek Pollard, Apr 22 '19 at 19:01
see this question https://stackoverflow.com/questions/31235599/array-length-and-undefined-indexes — bryan60, Apr 22 '19 at 19:02
Possible duplicate of [How to insert an item into an array at a specific index (JavaScript)?](https://stackoverflow.com/questions/586182/how-to-insert-an-item-into-an-array-at-a-specific-index-javascript) — Heretic Monkey, Apr 22 '19 at 19:03
`push` is an `Array.prototype` method. You can not use it on non Array values such as `undefined`. — Redu, Apr 22 '19 at 20:58

Khalid Ali · Accepted Answer · 2019-04-22T21:28:10.167

Here's a quick way to do exactly what you want.

var arr = [];

(arr[0]=[]).push([1,2]);

console.log(arr)

Also it's safer to check if an array already exists at this index, otherwise it will be replaced with an empty array – the above code will replace the array with an empty one if it already exists.

// creates an array at index 0 only if there's no array at this index.
// pushes an array of [1, 2] to the existing array at 0 index
// or the newly created empty one.
(arr[0] || (arr[0] = [])).push([1, 2]);;

var arr = [];

arr[0] = [1];

(arr[0]||(arr[0]=[])).push(2,3);

console.log(arr)

score 1 · Answer 2 · answered Apr 22 '19 at 19:03

1

You are going to have to set it to an empty array before you can push it.

neg[0] = neg[0] || []
neg[0].push([1,2]);

answered Apr 22 '19 at 19:03

epascarello

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ludovico · Answer 3 · 2019-04-22T19:28:18.150

Push will add elements to the end of an array. For example:

var neg = [];
neg.push(3); //this will add the number 3 to the end of your array

The problem here is your trying to add a value to an undefined element:

var neg = [];
neg[0].push([1,2]); // <-- neg[0] is not defined at this moment, and you're treating it as an array object.

If you define your elements as an array, you will not get such an error:

var neg = [];
neg[0] = [];
neg[22] = [];

neg[0].push([1,2]);
neg[22].push([1,2]);

Alternatively (and this is probably what you're probably looking for), you can set the value for any element of your array with the desired index:

var neg = [];

neg[0] = [1,2];
neg[22] = [1,2];

score 0 · Answer 4 · edited Jul 13 '22 at 23:32

0

You can specify how large the array will be if you know ahead of time, for example var neg = new Array(40); will create a new array with 40 undefined elements.

Once that is created, you can change the value at any index as long as 0 <= index <= 39.

edited Jul 13 '22 at 23:32

Dana Woodman

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answered Apr 22 '19 at 19:02

Jud

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well, unlike in other languages, in JavaScript you can define an array of an undefined length (```var myArray=[]```). Also, note that push will add elements to the end of the array (even beyond it's initial length) and returns the new length of the array --example here: https://jsbin.com/nuriqiqixa/edit?html,js,console,output – ludovico Apr 22 '19 at 19:25
`var neg = [40]` creates an array with one item with the value `40` not an array with `40` items. – Dana Woodman Jul 13 '22 at 23:31

score -1 · Answer 5 · answered Apr 22 '19 at 19:02

-1

You can use array.splice(your_index, 0, your_element); to push an element into an array at the given index.

answered Apr 22 '19 at 19:02

Schokokuchen Bäcker

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How to push new elements to an array with undefined index in JavaScript

5 Answers5