Question

I’ve found that on Opera 11.50 the expression

 JSON.stringify(2)

returns an object for which

• typeof returns "string"
• constructor.name is String
• charCodeAt(0) is 50
• length is 1

But still

alert(JSON.stringify(2) == "2")

shows “false” in Opera (and the same happens using ===).

Is this a bug or what?

The only way I found to make it compare equal to “2” is calling .substr(0) (for example even adding an empty string still compares different).

Answer 1

That definitely looks like a bug.

From the ECMAScript 5.1 specification:

Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in this specification without any deletions or extensions to the format. This differs from RFC 4627 which permits a JSON parser to accept non-JSON forms and extensions.

And:

JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a String that conforms to the JSON grammar

It may be that it somehow wraps the string in a “JSONText” type object which still has a typeof of string but that seems very odd.

I would definitely think that the following implementation in this case is the correct one:

JSON.stringify(2) == "2" && JSON.stringify(2) === "2" && JSON.stringify(2) == 2 && JSON.stringify(2) !== 2;
true

According to @6502 (see comment) this is true in:
Chrome; Firefox; IE9; iPad Safari; OsX Safari; the N1 Android browser

The ECMAScript 5.1 specification document: http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf