In modern browsers you can use Object.getOwnPropertyNames to get all properties (both enumerable and non-enumerable) on an object. For instance:
function Person ( age, name ) {
this.age = age;
this.name = name;
}
Person.prototype.greet = function () {
return "My name is " + this.name;
};
Person.prototype.age = function () {
this.age = this.age + 1;
};
// ["constructor", "greet", "age"]
Object.getOwnPropertyNames( Person.prototype );
Note that this only retrieves own-properties, so it will not return properties found elsewhere on the prototype chain. That, however, doesn't appear to be your request so I will assume this approach is sufficient.
If you would only like to see enumerable properties, you can instead use Object.keys. This would return the same collection, minus the non-enumerable constructor property.