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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 

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

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.

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