Rectangle 27 1

What is the JavaScript operator and how do you use it?

1>>>0            === 1
-1>>>0           === 0xFFFFFFFF          -1>>0    === -1
1.7>>>0          === 1
0x100000002>>>0  === 2
1e21>>>0         === 0xDEA00000          1e21>>0  === -0x21600000
Infinity>>>0     === 0
NaN>>>0          === 0
null>>>0         === 0
'1'>>>0          === 1
'x'>>>0          === 0
Object>>>0       === 0

(*: well, they're defined as behaving like floats. It wouldn't surprise me if some JavaScript engine actually used ints when it could, for performance reasons. But that would be an implementation detail you wouldn't get to take any advantage of.)

(In reality there's little practical need for this as hopefully people aren't going to be setting array.length to 0.5, -1, 1e21 or 'LEMONS'. But this is JavaScript authors we're talking about, so you never know...)

+1 for array.length = LEMONS. Have to find a really annoying person so I can play this trick on their code :3

+2 in depth description and table, -1 because array.length validates itself and can't be arbitrarily set to anything that is not an integer or 0 (FF throws this error: RangeError: invalid array length).

Although JavaScript's Numbers are double-precision floats(*), the bitwise operators (<<, >>, &, | and ~) are defined in terms of operations on 32-bit integers. Doing a bitwise operation converts the number to a 32-bit signed int, losing any fractions and higher-place bits than 32, before doing the calculation and then converting back to Number.

Great explanation and great examples! Unfortunately this is another insane aspect of Javascript. I just don't understand what's so horrible about throwing an error when you receive the wrong type. It's possible to allow dynamic typing without allowing every accidental mistake to create a type casting. :(

However, the spec deliberately allows many Array functions to be called on non-Array (eg. via, so array might not actually be a real Array: it might be some other user-defined class. (Unfortunately, it can't reliably be a NodeList, which is when you'd really want to do that, as that's a host object. That leaves the only place you'd realistically do that as the arguments pseudo-Array.)

In this case this is useful because ECMAScript defines Array indexes in terms of 32 bit unsigned ints. So if you're trying to implement array.filter in a way that exactly duplicates what the ECMAScript Fifth Edition standard says, you would cast the number to 32-bit unsigned int like this.

It doesn't just convert non-Numbers to Number, it converts them to Numbers that can be expressed as 32-bit unsigned ints.

Oh, and @yoshi, please don't! Let's not scare off those trying to learn...

So doing a bitwise operation with no actual effect, like a rightward-shift of 0 bits >>0, is a quick way to round a number and ensure it is in the 32-bit int range. Additionally, the triple >>> operator, after doing its unsigned operation, converts the results of its calculation to Number as an unsigned integer rather than the signed integer the others do, so it can be used to convert negatives to the 32-bit-two's-complement version as a large Number. Using >>>0 ensures you've got an integer between 0 and 0xFFFFFFFF.

Rectangle 27 1

What is the JavaScript operator and how do you use it?

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

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