Addition Assignment Operator (+=) (JavaScript)

Adds the value of an expression to the value of a variable and assigns the result to the variable.


result += expression   


Any variable.

Any expression.


Using this operator is exactly the same as specifying: result = result + expression.

The types of the two expressions determine 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


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.

See Also

Addition Operator (+)
Operator Precedence
Operator Summary (JavaScript)