JavaScript tutorial:
-= Operator 

The (-=) operator is used to subtract the value of an expression from a variable.

Syntax

result -= expression

The -= operator syntax has these parts:

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

Example

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.

See also: - Operator, Operator Behavior, Operator Precedence, Operator Summary