To demonstrate:

'tan' in Math;                            // true
Math.hasOwnProperty('tan');               // true
Object.hasProperty(Math, 'tan');          // true

Math.tan = void(0)

'tan' in Math;                            // true
Math.hasOwnProperty('tan');               // true
Object.hasProperty(Math, 'tan');          // false

Although it can’t handle contrived sabotage; I think the revised hasProperty is ‘good enough’ and I can’t see how it could be improved.

Object.hasProperty = function(object, property) {
	if (typeof(object[property]) != 'undefined') {
		return true;
	}

	for (var p in object) {
		if (p == property) {
			return true;
		}
	}

	return false;
};

Object.hasProperty({}, 'toString');       // true
Object.hasProperty([], 'length');         // true

({}).hasOwnProperty('toString');          // false
[].hasOwnProperty('length');              // true

Object.hasProperty({a: void(0)}, 'a');    // true
Object.hasProperty({}, 'undefined');      // false

Márcio, your Object.hasProperty function is closer in function to hasOwnProperty than to in.

The difference is clear when checking for a ‘length’ property on an array, or checking for ‘toString’ on any object:

'toString' in a;   // true
Object.hasProperty(a, 'toString');  // false

'length' in []; // true
Object.hasProperty([], 'length'); // false
// but... this isn't the same as hasOwnProperty either:
[].hasOwnProperty('length'); // true

As I said before, I don’t think there’s a perfect fix; but I’m sure there are cases where your hasProperty function could come in handy.

(The differences between in used as an operator, and in used in a for-in statement are defined in sections 11.8.7 and 12.6.4 of the ECMAScript Language Spec.)

Object.hasProperty = function(object, property) {
	for (var p in object) {
		if (p == property) {
			return true;
		}
	}

	return false;
};

For example:

var a = {b: void(0)};

typeof(a.c);    // 'undefined'
typeof(a.b);    // 'undefined'

'c' in a;       // false
'b' in a;       // true

// Cross-browser:
Object.hasProperty(a, 'c');    // false
Object.hasProperty(a, 'b');    // true
delete a.b;
Object.hasProperty(a, 'b');    // false

Also, don’t forget you can use the syntax: in , instead of hasOwnProperty.

You could, but that still doesn’t guarantee the property methodName is unused (an undefined value is perfectly legitimate.)

With modern browsers, you can check for the existence of an object property using hasOwnProperty. With your code, you’d change the while-condition to while (thisArg.hasOwnProperty(methodName))

However, IE5 doesn’t support hasOwnProperty, and I don’t think there’s a perfect fix for it.