mvf.js

gistfile1.js

var M = function() { this.m = function() { return 42 } };

var inst = new M();

inst.f = function() { return 42 };

// How to tell that f is a function in a field and 
// m is a method in an *arbitrary* object?
// 
// example:

is_method(inst.f);
//=> false

is_method(inst.m);
//=> true

// Is it possible to write is_method?

Yes, if I control the construction mechanism of all instances that I care about then this is a trivial problem. However, the real question is if it's possible to do so for any object.

It can't be done. Methods have no idea that they are special in any way from regular functions. Here is a modified version of what you have written to demonstrate the point.

var M = function() { this.m = function() { return this.r }; this.r = 42 };
var inst = new M();
var f = inst.m;
inst.m(); // => 42
f(); // => undefined
var r = 45;
f(); // => 45

The way I understand things is that "this" is always bound dynamically each time a function is called unless you use bind to fix it to a specific object. So there is no way to get the context of the function and see if it's bound to anything and distinguish it from a regular function.

In your example, the method assignments are functionally equivalent -- this in the constructor is going to be == your inst; the interpreter doesn't keep track of ("doesn't care") whether the value at a given field is assigned as part of the constructor's operation, or if it's assigned later. So the short answer is: "No, you can't write an is_method."

What you're looking for is probably something more like this:

M.prototype.m = function() { return this.v; };

In your example, the method assignments are functionally equivalent -- this in the constructor is going to be == your inst; the interpreter doesn't keep track of ("doesn't care") whether the value at a given field is assigned as part of the constructor's operation, or if it's assigned later. So the short answer is: "No, you can't write an is_method."

What you're looking for is probably something more like this:

var M = function() { this.v = 42; };
M.prototype.m = function() { return this.v; };

var inst = new M();

inst.f = function() { return this.v; };

inst.hasOwnProperty('f');
// true

inst.hasOwnProperty('m');
// false