Javascript array alllocation [duplicate]

Question

I was dealing with a problem and got to the point where I thought I could use the Ids to my entity instances as array indices for easy lookup.

var myArray = [];
myArray[obj.Id] = true;

Assume obj.Id is 1000 here, so will be myArray.length. Am I allocating 1000 bytes for a single boolean value here or is it just returning the maximum index as length?

Why not use a key value style object, var myArray = {}; myArray[obj.Id] = true; — Arun P Johny
– Arun P Johny, Commented Mar 29, 2016 at 5:08
it's "returning the maximum index as length", but sparse arrays slow down [].map() and for(i=0;i<myArray.length;i++)-style loops... — dandavis
– dandavis, Commented Mar 29, 2016 at 5:16

Arun P Johny · Accepted Answer · 2016-03-29 05:11:59Z

2

It won't be allocating so many bytes.

But what you are really looking for is an key-value object like

var myObj = {};
myObj[obj.Id] = true;
//then to access
console.log(myObj[obj.Id])

answered Mar 29, 2016 at 5:11

Arun P Johny

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2 Comments

Vahid Over a year ago

Thanks. Yes I did use the object approach eventually. I was just interested to know if array space allocation is based on index.

Vahid Over a year ago

And could you please explain why is using objects rather than array is a better approach then?

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