Javascript: Array is not empty but size is 0

Question

I create a array in Javascript. I want to use the array for key-value pair.

I can successfully add new items and delete items but the length of it is always 0.

Actually the problem I faced is when I want to convert it to a JSON string, it shows empty string: "[]". I just wonder if they are related. And I would also want to know why I cannot convert it.

Below is the debug information, It show the array contain 3 objects but the length is 0. The browser is Firefox 44.0 for Ubuntu.

Thanks!

Fellow @georg idea, I limited the index into 1000 and leave the rest code as before. Everything works including the JSON part. The next picture shows that in my array, there are actual 4 elements, the maximum index is 805.

One thing I learned is that the length of Javascript Array is not the real size but the maximum index number.

The newest finding, the array is really as long as the largest index number. The rest of the element is Null.

I think you're better using an object than an array, can you please post the object structure? — NMunro
– NMunro, Commented Jan 29, 2016 at 13:57
How are you adding the items? We can't tell you why it's not working if you don't let us see where you're going wrong. — krillgar
– krillgar, Commented Jan 29, 2016 at 13:58
objectsinfo should be an object ({}) instead of an array ([]) — Andreas
– Andreas, Commented Jan 29, 2016 at 14:04
@georg the question originally didn't have the image included inline, which was integral to the understanding the problem, without sufficient any explanation in the text. (Of course, the OP couldn't include the image, lacking sufficient rep to do so.) With the image included, it's indeed a very nice question now. — apsillers
– apsillers, Commented Jan 29, 2016 at 15:42

georg · Accepted Answer · 2016-01-29 14:18:00Z

16

This is one of many oddities in javascript.

> a = []
[]
> a[4294967295] = 1
1
> a.length
0 // wtf?

The problem is that Array.length is limited to 32 bits, so its max value is 2^32-1 = 4294967295. If you provide an index greater than or equal to 4294967295, the engine creates a new entry in the array, as usual, but refuses to update its length - without throwing any error. So you end up with a bag of properties, and the length equal to 0.

As others suggested, you will be better off using objects if you have such large numeric indexes.

answered Jan 29, 2016 at 14:18

georg

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2 Comments

Pingjiang Li Over a year ago

That is the correct answer! Thanks! Sorry I don't have 15 reputation yet.

georg Over a year ago

@PingjiangLi: glad to hear it helped. Don't worry about the reps.

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