Question

Both firefox’s js engine/gjs (spidermonkey) and webkit’s jscore seems to have different behavior when calling builtin and normal js functions on non-object.

gjs> toString.call(null)
"[object Null]"
gjs> function cccc() {return toString.call(this);}
gjs> cccc.call(null)
"[object GjsGlobal]"

So if a normal js function is called with non-object, its this is automatically get replaced by the current this, whereas this won’t happen for a builtin function.

Is this the standard behavior (according to some specification?) or is it just a implementation dependent behavior?
Is the first line always safe to check the type of any value?

THX

Answer 1

That behaviour is by design. As noted in the Mozilla documentation:

Note that this may not be the actual value seen by the method: if the
method is a function in non-strict mode code, null and undefined will
be replaced with the global object, and primitive values will be
boxed.

Apparently, builtin functions are (treated as) strict mode code. Your own functions are not, unless, of course, you write a strict mode function. 🙂

This will be different in ECMAScript 5. From the specification (PDF):

NOTE The thisArg value is passed without modification as the this
value. This is a change from Edition 3, where a undefined or null
thisArg is replaced with the global object and ToObject is applied to
all other values and that result is passed as the this value.

Here’s a more direct link to a non-normative but HTML version of the spec.