Clean Code in JavaScript

The addition operator

The addition operator is a dual-purpose operator:

• If either operand is String, then it'll concatenate both operands together
• If neither operand is String, then it'll add both operands as numbers

To accomplish its dual purpose, the + operator first needs to discern whether the operands you've passed can be considered strings. Obviously, a primitive String value is clearly a string, but for non-primitives, the + operator will attempt to convert your operands into their primitive representations by relying on the internal ToPrimitive procedure that we detailed in the last chapter, in the Conversion to a primitive section. If the output of ToPrimitive for either of our + operands is a string, then it will concatenate both ...