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JavaScript: The Good Parts by Douglas Crockford

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+

The + operator can add or concatenate. Which one it does depends on the types of the parameters. If either operand is an empty string, it produces the other operand converted to a string. If both operands are numbers, it produces the sum. Otherwise, it converts both operands to strings and concatenates them. This complicated behavior is a common source of bugs. If you intend + to add, make sure that both operands are numbers.

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