What good does zero-fill bit-shifting by 0 do? (a >>> 0)

Question

I just came across this piece in the Mozilla Javascript documentation:

var len = this.length >>> 0;  

I don't quite understand why this is being done. What good does zero-fill right shifting this.length by 0 do? As far as I understand, it does exactly nothing. Is it to safely establish a default value for len, even if this.length is not an integer? Can this ever feasibly be the case? If so, what's the difference between >> 0 and >>> 0?

Answer 1

>>> is the unsigned right shift operator (see p. 76 of the JavaScript 1.5 specification), as opposed to the >>, the signed right shift operator.

>>> changes the results of shifting negative numbers because it does not preserve the sign bit when shifting. The consequences of this is can be understood by example, from an interpretter:

$ 1 >> 0
1
$ 0 >> 0
0
$ -1 >> 0
-1
$ 1 >>> 0
1
$ 0 >>> 0
0
$ -1 >>> 0
4294967295
$(-1 >>> 0).toString(16)
"ffffffff"
$ "cabbage" >>> 0
0

So what is probably intended to be done here is to get the length, or 0 if the length is undefined or not an integer, as per the "cabbage" example above. I think in this case it is safe to assume that this.length will never be < 0. Nevertheless, I would argue that this example is a nasty hack, for two reasons:

1. The behavior of <<< when using negative numbers, a side-effect probably not intended (or likely to occur) in the example above.

2. The intention of the code is not obvious, as the existence of this question verifies.

Best practice is probably to use something more readable unless performance is absolutely critical:

isNaN(parseInt(foo)) ? 0 : parseInt(foo)

Answer 2

Two reasons:

1. The result of >>> is an "integral"

2. undefined >>> 0 = 0 (since JS will try and coerce the LFS to numeric context, this will work for "foo" >>> 0, etc. as well)

Remember that numbers in JS have an internal-representation of double. It's just a "quick" way of basic input sanity for length.

However, -1 >>> 0 (oops, likely not a desired length!)