[JavaScript] LGTM.com - false positive for logical OR assignment

[timvandermeij]

Description of the false positive

Mozilla's PDF.js project recently integrated CodeQL and it doesn't seem to understand logical OR assignment, or the ||= operator. This operator is explained on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment. On the link below LGTM states that the externalListeners variable is never set, but it clearly is on line https://lgtm.com/projects/g/mozilla/pdf.js/snapshot/02cf4948474989afc65c0b58414fc2dc379119bc/files/web/ui_utils.js?sort=name&dir=ASC&mode=heatmap#L782.

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/mozilla/pdf.js/snapshot/02cf4948474989afc65c0b58414fc2dc379119bc/files/web/ui_utils.js?sort=name&dir=ASC&mode=heatmap#x9a849b47d4d84b8e:1

[timvandermeij]

For reference, this is not LGTM-specific since we now integrated with GitHub Advanced Security and that shows the same issue (presumably because both use the same CodeQL ruleset).

[erik-krogh]

You're right, that is a false positive.
I'm looking into it.

Here is a minimal example that triggers the false positive.

function f() {
    let foo;
    if (bar) foo ||= [];
    if (foo);
};

[erik-krogh]

The FP should be fixed with #6459

If might take a few weeks for the fix to appear on LGTM / GHAS.

Again, thanks for the report.
The fix also fixed a few other FPs in some other projects we hadn't noticed.

[timvandermeij]

Glad to help, and thank you for fixing this so quickly!