1436822 - Startup terminalspew: "JavaScript error: chrome://browser/content/tabbrowser.xml, line 2473: TypeError: cannot use 'in' operator to search for 'canGoBack' in 'browser'"

Status: RESOLVED DUPLICATE of bug 1436351

(Firefox :: General, defect)

Description

[Daniel Holbert [:dholbert]]

Attachment: screenshot of browser console with source code highlighted & callstack

I noticed this today, among the first ~20 lines of terminal output of my local mozilla-central debug build, when I run e.g. "./mach run" or "./mach reftest":

> JavaScript error: chrome://browser/content/tabbrowser.xml, line 2473:
>  TypeError: cannot use 'in' operator to search for 'canGoBack' in 'browser'

It shows up in the Browser Console, too (see screenshot coming up).  It seems to point to this chunk of code:

>            // If browser is a lazy browser, delete the substitute properties.
>            if (this._browserBindingProperties[0] in browser) {
>              for (let name of this._browserBindingProperties) {
>                delete browser[name];
>              }
>            }

I think it's complaining about the first line there (the "this._browserBindingProperties[0] in browser" check), though technically the reported line number is for the following line.

Not sure what's going on here, but I'm pretty sure this is a recent regression, because I don't recall seeing this error output before.

Comment 1

[Daniel Holbert [:dholbert]]

My tree is based on https://hg.mozilla.org/mozilla-central/rev/0ac953fcddf1 (with some unrelated gecko layout patches stacked on top).

Comment 2

[Daniel Holbert [:dholbert]]

(and I'm on 64-bit Ubuntu 17.10 Linux, if it matters)

Comment 3

[Daniel Holbert [:dholbert]]

Ah, looks like this was a dupe of bug 1436351, which has since been fixed -- hooray!

I confirmed that I don't see this in my startup terminal output anymore, too. (So it does seem indeed to be fixed.)