sstephenson/prototype — comment on commit 460be4691af9e4e92908c9ac707597bad69be57c
File: src/prototype/lang/array.js line 517
By savetheclocktower:

I had to look up what it was — it's a "zero-fill right shift" bitwise operator. You're shifting the number zero bits to the right, but also resetting the sign bit to `0`, so negative numbers come out positive. It appears to constrain an integer to the lower bound of `0` and the upper bound of `2^32 - 1`. It also coerces (e.g.)`'3'` to `3`, but nearly everything else (objects, `null`, `undefined`) gets coerced to `0`. So, by a combination of intent and sheer coincidence, it ends up being equivalent to the [ToUInt32](http://es5.github.com/#x9.6) operation described in the ES5 spec.

First place I saw it was in all the MDN polyfills for array extras ([example](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/filter#Compatibility)). It does feel a bit magical, and I'm not wild about that, but it saves me from having to implement the crazy conversion rules in the spec, so it's a dear friend of mine.