CODEWITHSUNDEEP
javascript
Jacksonkr
Jacksonkr

Reputation: 32207

Javascript insane memory woe with Array

Exhibit A:

var pt = [0,0,0,0,0,0];
console.log(pt[0], pt);

sometimes the log will spit out 0 [Infinity, Infinity, Infinity, Infinity, Infinity, Infinity], but only when you do a SIX zeroes array for pt. If you do five zeroes then you get an array of five zeroes as expected. The big clencher is how pt[0] can occassionally be 0 and Infinity a the same time confusion

Steps to re-create the problem:

*Notes

If there are any problems with my instructions, please post back and I'll get to them ASAP. Thanks.

** Clarification of the question **

If I run console.log(pt[0], pt); then I expect that the first value from the array pt and pt[0] will be identical. That's not the case in my Exhibit A. Why not?

Upvotes: 2

Views: 152

Answers (1)

RightSaidFred
RightSaidFred

Reputation: 11327

console.log() isn't necessarily synchronous, so whatever code is causing the values to be Infinity is likely happening later.

If you want to see the Array values in their current state, you need to be sure to capture its current value somehow.

One way is to abuse JSON for this purpose.

var pt = [0,0,0,0,0,0];
console.log(pt[0], JSON.stringify( pt ));

Or since it's an Array of primitives, you could slice it.

var pt = [0,0,0,0,0,0];
console.log(pt[0], pt.slice());

To find the actual issue, you're going to have to follow pt along and see who's misusing it.

demonstration

Here's a demo for illustrative purposes.

http://jsfiddle.net/J7tbB/

var later = Date.now() + 1000;

var arr = [0,0,0,0,0];

console.log( arr );

while( Date.now() < later ) {
    var len = arr.length;
    while( len-- ) arr[len]++;
}

Note that this freezes your browser for 1000ms.

As you can see, it creates an Array with 5 members initialized at 0, then immediately passes the Array to console.log.

Right after that you have a while loop that will run for 1000ms, incrementing each member once for each iteration.

If the console.log was synchronous, you'd get your zeros. Instead you'll likely get something like:

[2811349, 2811349, 2811349, 2811349, 2811349]

Now if we change it so that it logs a .slice() of the Array, you'll see that you get your expected values.

http://jsfiddle.net/J7tbB/1/

console.log( arr.slice() );

// ...

[0, 0, 0, 0, 0]

Upvotes: 4

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