Javascript ++ vs +=1

Question

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

Answer 1

++ converts to number, and then increments, += with a String concatenates.

From the spec:

11.3.1 Postfix Increment Operator

  ...
  3. Let oldValue be ToNumber(GetValue(lhs)).
  4. Let newValue be the result of adding the value 1 to oldValue, 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. If Type(lprim) is String or Type(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).

Answer 2

• ++ 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.