new Array(length) gives wrong size

Question

Update: This bug affects v28 and was fixed for v29 onward.

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Having a function like this:

function arrMany(len) {
    var a = new Array(len);
}

If the function is called rapidly the length of the produced array sometimes has length of previous call to the function.

Expanding the function to:

function arrMany(len, show) {
    var a = new Array(len);
    if (show) {
        someElementTextA.value = a.length;
        someElementTextB.value = len;
    }
}

And calling by e.g.:

function tick() {
    arrMany(2);
    arrMany(4);
    arrMany(6);
    arrMany(10, true);
}

setInerval(tick, 1000 / 200);

after a while someElementTextA shows the value 6 and not 10.

• Sample fiddle edit
• Sample fiddle show

When this happens, and one turn down fps to for example 1, it keeps spitting out wrong length for a long time.

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The behavior typically manifests itself after about 20 seconds on my system.

Tested on: Firefox Linux i686; rv:28.0

Notes:

1. No action in Browser Console or Web Console.
2. If Web Console is open the error never occurs.
3. If Web Console is opened when the error manifests, it corrects itself at once.
4. Given pt. 3. it is rather hard to debug in a sane way using the browser itself.

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Any good fix to this?

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Update 1:

Have tested this on Firefox, Chrome and Opera. Firefox is the only browser that manifests the bug. No action in console.

From comments and answer: Does not manifest on Window XP, Firefox v31. Does manifest on @jdigital's system – i.e. it's not local to my system.

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Update 2:

Always specifying show in above code does not make any difference.

function tick() {
    arrMany(2, false);
    arrMany(4, false);
    arrMany(6, false);
    arrMany(10, true);
}

Nor does using default parameter. But another point being that show is only for the convenience of the sample code.

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As to solution for conundrum saying:

if (arr.length !== len) { arr = new Array(len); }

seems to work. But not sure if is is a fix, or if this can break the same way. Using

if (arr.length !== len) { arr = new Array(len); retry = 1; }

show that retry is done quite frequent.

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Another fix, (as in debug testing); but in another way. By adding this inside the function, the array is always of correct length. As in: retry is always 0.

for (retry = 0; arr.length !== len && retry < 10; ++retry) {
    arr = new Array(len);
}

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As to question.

• Question is more if there is any good fix to the bug, not if it is a bug or not.

Any information for why this is happening and if there is any similar code, e.g. not handling Arrays, that can break in the same way is also interesting, but was not part of the original question as such.

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Update 3:

For, "what I am trying to do", it is a question about it in the general sense, not specific to a case.

It is not a syntax I usually use, but happens from time to time, usually in some debug or verbose output where using a sparse array is simpler then a loop. But it is then nice to know that it is not reliable.

At the same time I recognize others might have different needs and as such solutions to it, in the broad general sense, is always nice to have.

Then again; tipping it the other way around I guess it would be another argument against ever using it ... ;)

Answer 1

This worked for me:

function arrMany(w, show) {
    (function () {
        var a = new Array(w);
        if (show) {
            arw1.value = a.length;
            arw2.value = w;
        }
    })();
}

Interesting read: JavaScript's Strange Threaded Nature

This code explores the following property:

(function() { new Object(); })(); is garbage collected properly

Answer 2

It looks like a bug in Firefox. This problem does not occur in Chrome etc.
When I add a logging statement:

function arrMany(w, show) {
    var a = new Array(w);
    if (show) {
        if( a.length != w ) {
            console.log(a);
        }
        arw1.value = a.length;
        arw2.value = w;
    }
}

When I run this, I can see that console.log is occasionally triggered, and the Firefox Web Console window shows that the array has 5 elements.

Answer 3

If I understand correctly, you want a workaround for this bug, even if it is an ugly hack, as long as it is reliable. If so, consider this approach, which explicitly initializes the array:

function arrMany(w, show) {
    // should be:
    //      var a = new Array(w);
    // but there's a bug in Firefox, see StackOverflow 22726716
    //
    var a = [];
    for( var i = 0; i < w; i++ ) a[i] = undefined;

    if (show) {
        arw1.value = a.length;
        arw2.value = w;
    }
}