Why does legacy content break when making array-likes real arrays?

Erik Arvidsson erik.arvidsson at gmail.com
Sun Dec 30 14:50:15 PST 2012

The problem was code that incorrectly assumed instanceof to mean that the
instance was an Array in combination with Array.prototype.concat.

There was a bug in Google's Closure Library which caused almost all
Google's apps to fail due to this. The library was updated as soon as this
was found but there might still be code out there that makes the same
incorrect assumption in combination with concat.

I wanted to make another try under a runtime flag in Chromium but was
blocked by Apple.
On Dec 30, 2012 5:24 PM, "Domenic Denicola" <domenic at domenicdenicola.com>

> > From: es-discuss-bounces at mozilla.org [mailto:
> es-discuss-bounces at mozilla.org] On Behalf Of Anne van Kesteren
> > Sent: Sunday, December 30, 2012 17:18
> >
> > On Sun, Dec 30, 2012 at 10:22 PM, Axel Rauschmayer <axel at rauschma.de>
> wrote:
> > > 5. Array-like objects [not completely fixed (DOM...), but `arguments`
> >
> > We've made some effort towards changing some of this, but legacy content
> is against us. :/
> I saw this happen, and was sad, but never understood what the legacy
> problem was.
> Similarly, when people ask me "why can't they just make arguments a real
> array" (perhaps with magic aliasing behavior in sloppy mode), I don't know
> the answer.
> So just out of curiosity, how does legacy code break when making
> array-likes into real arrays? I feel like the Liskov Substitution Principle
> should have our back here.
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