Subtraction Assignment Operator (-=) (JavaScript)
JavaScript - Internet Explorer 10
Subtracts the value of an expression from the value of a variable and assigns the result to the variable.
result -= expression
Supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards, Internet Explorer 9 standards, Internet Explorer 10 standards. Also supported in Windows Store apps. See Version Information.