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

String concatenation is done with + so Javascript will convert the first numeric 1 to a string and concatenate "1" and "1" making "11".

You cannot perform subtraction on strings, so Javascript converts the second "1" to a number and subtracts 1 from 1, resulting in zero.

+ is ambiguous. It can mean "concatenate" or "add". Since one side is a string, it is taken to mean "concatenate", hence the result is 11 (which, by the way, was one of my favourite jokes as a young child. That and "1 + 1 = window", as shown visually: │┼│ ニ ⊞)

- however has only one meaning: subtract. So it subtracts.

This kind of problem is not present in other languages such as PHP, where "concatenate" is . instead of +, making no ambiguity. Still other languages like MySQL don't even have a concatenation operator, instead using CONCAT(a,b,c...).

Because the spec explicitly tells to do so. Page 75. Note the difference between 11.6.1 steps 5-8 and 11.6.2 steps 5-7.

11.6.1 - describes how addition operator works

1-4. ...

5. Let lprim be ToPrimitive(lval).

6. Let rprim be ToPrimitive(rval).

7. If Type(lprim) is String or Type(rprim) is String, then

7a. Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

8. Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim)

11.6.2 - describes how subtraction operator works

1-4. ...

5. Let lnum be ToNumber(lval).

6. Let rnum be ToNumber(rval).

7. Return the result of applying the subtraction operation to lnum and rnum

Summary In case of addition if any of the operands when converted to primitive value without any hints suddenly becomes a string the second one is converted to a string too. In case of subtraction both operands are converted to a number.