How does toString() work in javascript?

Question

"J" + { toString: function() { return "S"; } }; // "JS"

Why is the output "JS?"

When I do:

 "J" + { someFoo: function() { return "S"; } }; // "J[object Object]"

Why isn't this also "JS"?

I'm trying to figure out how the .toString() is getting used inside the first block.

Thanks

Answer 1

toString is a special function (in object's prototype), which is called when a stringified mode of the object is required.

In your cases, the addition operator calls the toString method of the object. From the specs :

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).

However, this native method can be overridden, which you've done* in the first snippet. Addition operator just calls the custom method, which produces the result you've got.

In the latter snippet toString just returns a default value for objects.

You can see this happening in many situations, for example alert({}) calls internal toString method from object's prototype, since alert requires a string as an argument.

(* More accurate: you haven't re-written the native property, rather you've created an own property to the object with the same name, which is used instead of searching the native property from the prototype chain.