Note that it doesn't matter whether the first operand is a string or the second one. The rule is simple: if any of operands is a string, then convert the other one into a string as well.

The string concatenation and conversion is the special feature of the binary plus `"+"`. Other arithmetic operators work only with numbers. They always convert their operands into numbers.

For instance:

alert( 2 - '1' ); // 1

alert( 6 / '2' ); // 3