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

Used to shift the bits of an expression to the right, preserving sign.
result >>= expressionThe >>= operator syntax has these parts:
Part Description result Any variable. expression Any expression.
Using the >>= operator is exactly the same as specifying:result = result >> expressionThe >>= operator shifts the bits of result right by the number of bits specified in expression. The sign bit of result is used to fill the digits from the left. Digits shifted off the right are discarded. For example, after the following code is evaluated, temp has a value of 4: 14 (11110010 in binary) shifted right two bits equals 4 (11111100 in binary).
var temp temp = 14 temp >>= 2For information on when a runtime error is generated by the >>= operator, see the Operator Behavior table.