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

Used to shift the bits of an expression to the left.
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 left by the number of bits specified in expression. For example:
var temp temp = 14 temp <<= 2The variable temp has a value of 56 as 14 (00001110 in binary) shifted left two bits equals 56 (00111000 in binary). Bits are filled in with zeroes when shifting.
For information on when a runtime error is generated by the <<= operator, see the Operator Behavior table.