Microsoft JScript
+= Operator
Language Reference
Version 1

See Also

Used to increment a variable by a specified amount.
result += expression

The += operator syntax has these parts:

Part Description
result Any variable.
expression Any expression.
Using this operator is exactly the same as specifying:

result = result + expression

The underlying subtype of the expressions determines the behavior of the += operator.
If Then
Both expressions are numeric or Boolean Add.
Both expressions are strings Concatenate.
One expression is numeric and the other is a string Concatenate.

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