Recently, Special:MentorDashboard broke because of an eslint update (see T372164#10057975 for details). For mentor dashboard specifically, this happened because of differences between $.extend and Object.assign:

> $.extend({a: 1, b: 2}, {a: undefined, b: 1})
{a: 1, b: 1}
> Object.assign({a: 1, b: 2}, {a: undefined, b: 1})
{a: undefined, b: 1}

1057374: build: Update eslint-config-wikimedia to 0.28.2 is a large patch, containing several other changes of $.extend with Object.assign. We should review those entries for possible similar issues. If possible, we should also look at other changes, in case they are non-equivalent (similarly to this case).