Adds the value of an expression to the value of a variable and assigns the result to the variable.
result += expression
Using this operator is exactly the same as specifying:
result = result + expression.
The types of the two expressions determine the behavior of the
|Both expressions are numeric or Boolean||Add|
|Both expressions are strings||Concatenate|
|One expression is numeric and the other is a string||Concatenate|
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, Internet Explorer 11 standards. Also supported in Store apps (Windows 8 and Windows Phone 8.1). See Version Information.