Adds the value of an expression to the value of a variable and assigns the result to the variable.
Using this operator is exactly the same as specifying:
The underlying subtype of the expressions determines the behavior of the += operator.
Both expressions are numeric or Boolean
Both expressions are strings
One expression is numeric and the other is a string
Supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards, Internet Explorer 9 standards. See Version Information.