Microsoft® JScript®
-= Operator
 Language Reference 
Version 1 

See Also

Used to subtract the value of an expression from a variable.
result -= expression

The -= operator syntax has these parts:

Part Description
result Any numeric variable.
expression Any numeric expression.

Using the -= operator is exactly the same as doing the following:
result = result - expression

For information on when a run-time error is generated by the - operator, see the Operator Behavior table.