Why does `Object.prototype.toString` always return `[object *]`?

Question

If you call Object.prototype.toString.call(anything) the result is always [object Something], where Something could be one of several

Answer 1

Annex F of the ES5.1 specification says this about Object.prototype.toString:

15.2.4.2: Edition 5 handling of undefined and null as this value caused existing code to fail. Specification modified to maintain compatibility with such code. New steps 1 and 2 added to the algorithm.

This is a correction that was made in the ES5.1 spec. Prior to ES5, passing null or undefined to toString always caused the global object to be passed instead. Strict mode related changes in ES5 cause null and undefined to be passed without modification. As specified in ES5, passing null or undefined to Object.prototype.toString caused a TypeError exception. This exception broke some existing code so we had to fix the spec to not throw in that case.

So what should toString return for null and undefined? It turns out that a lot of existing code also expects Object.prototype.toString to always return a string of the form "[object *]". So, we decided to make null and undefined produce "[object Null]" and "[object Undefined]".

This seems to have worked and generally permitted existing ES3-based code to keep working in the presence of the strict mode related changes in ES5/5.1.