CODEWITHSUNDEEP
javascriptarraysperformance
jeff
jeff

Reputation: 13663

What is a good way to create a JavaScript array with big indices?

I'm making a web app where a user gets data from PHP, and the data consists of MySQL rows, so I want to save the used ones in a global variable, something like a buffer, to prevent extra AJAX requests.

I'm doing this right now : 

window.ray = []; // global variable
$(function(){
   data = getDataWithAjax(idToSearch);
   window.ray[data.id] = data.text;
});

but when the id is big, say 10 for now, window.ray becomes this : 

,,,,,,,,42

so it contains 9 unnecessary spots. Or does it? Is it only visible when I'm doing console.log(window.ray);

If this is inefficient, I want to find a way like PHP, where I can assign only indices that I want, like :

$array['420'] = "abc";
$array['999'] = "xyz";

Is my current way as efficient as PHP, or does it actually contain unnecessary memory spots?

Thanks for any help !

Upvotes: 0

Views: 49

Answers (2)

RobG
RobG

Reputation: 147413

If you are assigning values directly by property name, then it doesn't make any difference in terms of performance whether you use an Array or an Object. The property names of Arrays are strings, just like Objects.

In the following:

var a = [];
a[1000] = 'foo';

then a is (a reference to) an array with length 1,001 (always at least one greater than the highest index) but it only has one numeric member, the one called '1000', there aren't 1,000 other empty members, e.g.:

a.hasOwnProperty['999']; // false

Arrays are just Objects with a special, self–adjusting length property and some mostly generic methods that can be applied to any suitable object.

One feature of sparse arrays (i.e. where the numeric properties from 0 to length aren't contiguous) is that a for loop will loop over every value, including the missing ones. That can be avoided and significant performance gains realised by using a for..in loop and using a hasOwnProperty test, just like an Object.

But if you aren't going to use any of the special features of an Array, you might as well just use an Object as suggested by jfriend00.

Upvotes: 0

jfriend00
jfriend00

Reputation: 707546

Use an object instead of an array. The object will let you use the id as the key and be more efficient for non-sequential id values.

window.ray = {}; // global variable
$(function(){
   data = getDataWithAjax(idToSearch);
   window.ray[data.id] = data.text;
});

You can then access any element by the id:

var text = window.ray[myId];

Upvotes: 5

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