Microsoft® JScript™ >>>= Operator 
Language Reference Version 1

Used to make an unsigned right shift of the bits in a variable.
result >>>= expressionThe >>>= operator syntax has these parts:
Part Description result Any variable. expression Any expression.
Using the >>>= operator is exactly the same as doing the following:result = result >>> expressionThe >>>= operator shifts the bits of result right by the number of bits specified in expression. Zeroes are filled in from the left. Digits shifted off the right are discarded. For example:
var temp temp = 14 temp >>>= 2The variable temp has a value of 1073741820 as 14 (11111111 11111111 11111111 11110010 in binary) shifted right two bits equals 1073741820 (00111111 11111111 11111111 11111100 in binary).
For information on when a runtime error is generated by the >>>= operator, see the Operator Behavior table.