What is the >>>= operator in Javascript?

Question

What does the following Javascript statement do to a?

a >>>= b;

Answer 1

It does the same thing as this:

a = a >>> b;

Except that a is only evaluated once (which has observable difference if its evaluation involves any side effects).

And >>> is unsigned (logical) right shift.

Answer 2

It's a bitwise operator called zero-fill right shift. It will shift the binary representation of a to the right by b places, and replace the empty items with zeros. Then the result will be assigned to a.

Answer 3

I right shifts the value in a the number of bits specified by the value in b, without maintaining the sign.

It's like the >>= operator that rights shifts a value, only that one does not change the sign of the number.

Example:

var a = -1;

// a now contains -1, or 11111111 11111111 11111111 11111111 binary

var b = 1;
a >>>= b;

// a now contains 2147483647, or 01111111 11111111 11111111 11111111 binary.

Answer 4

Crockford points out that while JavaScript has bitwise operators like >>>, using them on its native double-precision floating point numbers implies converting back and forth to integers internally. They will not be as efficient as in other languages with native integer data types.