JavaScript triple greater than

Question

I saw this syntax on another StackOverflow post and was curious as to what it does:

var len = this.length >>> 0;

What does >>> imply?

Answer 1

Ignoring its intended meaning, this is most likely where you'll see it used:

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>>> 0 is unique in that it is the only operator that will convert any type to a positive integer:

"string"         >>> 0 == 0
(function() { }) >>> 0 == 0
[1, 2, 3]        >>> 0 == 0
Math.PI          >>> 0 == 3

In your example, var len = this.length >>> 0, this is a way of getting an integer length to use to iterate over this, whatever type this.length may be.

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Similarly, ~~x can be used to convert any variable into a signed integer.

Answer 2

That's an unsigned right shift operator. Interestingly, it is the only bitwise operator that is unsigned in JavaScript.

The >>> operator shifts the bits of expression1 right by the number of bits specified in expression2. Zeroes are filled in from the left. Digits shifted off the right are discarded.

Answer 3

That operator is a logical right shift. Here the number is shifted 0 bits. A shift of zero bits mathemetically should have no effect.

But here it is used to convert the value to an unsigned 32 bit integer.

Answer 4

>>> is a bit-wise operator, zero-fill right shift.

I think the only effect of >>> 0 on a positive number is to round down to the nearest integer, same as Math.floor(). I don't see why this would be necessary in your example, as generally a .length property (e.g. of an Array) would be an integer already.

I've also seen the slightly shorter ~~ used in the same way: ~~9.5 == 9; // true.