__proto__ : Spec & web compatibility

Suresh Jayabalan sureshja at microsoft.com
Wed Aug 21 17:38:21 PDT 2013

According to sections B.3.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-B.3.1>#6.a and<http://people.mozilla.org/~jorendorff/es6-draft.html#sec->#3, implementations are expected to throw a TypeError exception if an object's __proto__ is set with anything other than null or an object. Today the existing implementations (Chrome or Firefox) treat such assignments as a no op.

Interestingly there are instances of web pages who assign undefined to an objects __proto__ are found. For example yelp.com<http://www.yelp.com/biz/potbelly-sandwich-shop-seattle-4> assigns undefined to __proto__ via a function call as follows.

function(f) { return { __proto__:f } }

Implementing as per the specification would break the zoom in/out functionality of Yelp as this function would throw a TypeError. Similarly a radio player on myspace.com<http://myspace.com/> would not work either.  The fact that there are few instances we have seen in the wild would mean there could be more websites that could break.

Is the v8/spidermonkey behavior of silently ignoring primitive assignments to __proto__ a bug? Or should the spec mandate silently ignoring assignments of primitives (or just undefined) to __proto__?

- Suresh
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