How to insert an item into an empty array at a specific index?

Question

I want to insert a item to specified index of an empty Array. I saw there is Array.prototype.splice method. However, if I use splice on empty Array, it just add item to end off Array as shown below.

var a = [];
a.splice(3,0,"item-3");
console.log(a); //returns ['item-3']

What I want to accomplish is to have array as given below.

console.log(a); //returns [,,,'item-3']
or
console.log(a); //returns [undefined,undefined,undefined,'item-3']

Thanks for your help.

Edit: I saw the question on How to insert an item into an array at a specific index? but, it did not explain how to insert to specified index of empty array.

Do you mean a[3] = "item-3"? This is not a good idea to do this though. — Yeldar Kurmangaliyev
– Yeldar Kurmangaliyev, Commented Jan 4, 2019 at 12:21
Why it is not good idea to do ? I mean of course there will be undefined items but other than that is there any bad side ? @YeldarKurmangaliyev — ysnfrk
– ysnfrk, Commented Jan 4, 2019 at 12:33
Possible duplicate of How to insert an item into an array at a specific index? — Shahnewaz
– Shahnewaz, Commented Jan 4, 2019 at 12:37
@ysnfrk Because this is not how an array is supposed to be used. If you do var a = []; a[100000] = true;, then a.length will be 100001. Also, most of algorithms rely on this length, which can cause many problems. Just use object \ set if you need key-value association array. — Yeldar Kurmangaliyev
– Yeldar Kurmangaliyev, Commented Jan 4, 2019 at 14:29
@ysnfrk have you got any approach for this. I would like to know about it. — Amit Singh
– Amit Singh, Commented Jun 5, 2021 at 8:51

T.J. Crowder · Accepted Answer · 2019-01-04 12:23:13Z

18

Just use the index and do the assignment directly:

var a = [];

a[3] = "item-3";

console.log(a);

edited Jan 4, 2019 at 12:23

T.J. Crowder

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answered Jan 4, 2019 at 12:21

Nina Scholz

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1 Comment

T.J. Crowder Over a year ago

(As always: Hope my edit is helpful, but if not, please revert it.)

Simon Márton · Accepted Answer · 2019-01-04 12:25:40Z

6

Coming in for the ES6 solution:

[...Array(3), 'item-3']
// [undefined, undefined, undefined, "item-3"]

answered Jan 4, 2019 at 12:25

Simon Márton

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Comments

oFinally · Accepted Answer · 2021-04-01 09:37:46Z

0

Assign the value to the index like this:

var a = [];

a[3] = "item-3"; //3 is the index

console.log(a);

answered Apr 1, 2021 at 9:37

oFinally

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