var a = "ab"; var b = "ab"; a+=1; // "ab1" b++; // "NaN" (Tested on chrome's V8)
Can someone explain why the results are different based on the internal atomic actions of the ++ arithmetic operator and the += assignment operator with argument 1
++ converts to number, and then increments, += with a String concatenates.
From the spec:
11.3.1 Postfix Increment Operator
...
3. LetoldValuebeToNumber(GetValue(lhs)).
4. LetnewValuebe the result of adding the value 1 tooldValue, using the same rules as for the+operator (see 11.6.3).
For the a+=1 case, if you add a number to a string or the other way around the number gets converted to a string:
11.6.1 The Addition operator ( + )
...
7. IfType(lprim)is String orType(rprim)is String, then
a. 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).
+=1 case 1 is converted to a string for the concatenation.++ tries to increment a Number (if it's not a number, this will fail - resulting in NaN)+= is concatenation, in this case the JavaScript engine figures out that one side is a string, so they're both concatenated as strings.They're different because they're different operations, ++ is specifically an arithmetic operator, where as += is a more general assignment operator that behaves differently based on the data type - specifically, string has its own implementation.
++ & +=1 are logically equivelant. I guess I forgot + is overloaded for String & Number+ and += anyway, but it's more that ++ is specific to number here :)
x += 1 is ++x, not x++.That's because the + operator in javascript is both the mathematical + and the string concatenation operator, while the ++ is always a mathematical operator.
So, when you have:
string = string + number;
the number is converted to string and concatenated to the first string.
When you have
string++
you will convert the string to a number, getting NaN, and then add one to that - getting still, NaN.
a = '12';, and try++,--,+=1and-=1. JavaScript is fun.