Why does JavaScript handle the plus and minus operators between strings and numbers differently?

Question

I don\'t understand why JavaScript works this way.

console.log(\"1\" + 1);
console.log(\"1\" - 1);

The first line prints 11, and the second

Answer 1

According to the standard EcmaScript 262. The + and - operators behave differently when strings are involved. The first converts every value to a string. The second converts every value to a number.

From the standard:

If Type(lprim) is String or Type(rprim) is String, then Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

This rules implies that if in the expression there is a string value, all values involved in the + operation are converted to a string. In JavaScript when the + operator is used with strings, it concatenates them. This is why console.log("5"+1) returns "51". 1 is converted to a string and then, "5" + "1" are concatenated together.

Nevertheless, the above rule doesn't apply for the - operator. When you are using a - all values are converted to numbers according to the Standard (see below). Therefore, in this case, "5" is converted to 5 and then 1 is subtracted.

From the standard:

5 Let lnum be ToNumber(lval).

6 Let rnum be ToNumber(rval).

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Operator definition from the standard EcmaScript 262.

Operator + : http://www.ecma-international.org/ecma-262/5.1/#sec-11.6.1

Operator - : http://www.ecma-international.org/ecma-262/5.1/#sec-11.6.2