JavaScript tutorial:
+= Operator

Contents | JavaScript Language Reference

The (+=) operator is use to increment a variable by a specified amount.

Syntax

result += expression

The += operator syntax has these parts:

Part	Description
result	Any variable.
expression	Any expression.

Example

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.

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