How can I set the prototype when I don't know the name of the constructor?

Question

It was my understanding that every javascript object had a prototype even if it were just the one from Object.prototype

I have an asp.net webservice proxy that gets me participant data from the server. That part works fine. Here's the code:

        atomWebServiceProxy.GetInteractionParticipants(interactionId,
            function (participantsResult) {
                var assetId, pendingPathFetches;
                participants = participantsResult;
                pendingPathFetches = participants.length;
                document.title = participants.length + " participants:";
                for (var i = 0; i < participants.length; i++) {
                    assetId = participants[i].ASSETID;
                    document.title += " " + participants[i].DISPLAYID;
                    atomWebServiceProxy.GetInteractionPath(interactionId, assetId,
                        function (pathResult, participant) {
                            participant.path = pathResult;
                            pathResult.participant = participant;
                            if (--pendingPathFetches == 0) {
                                setTimeout(afterInit, 20);
                            }
                        },
                        function (error, participant) {
                            alert(participant.DISPLAYID + " reported this error: " + error.get_message());
                        },
                        participants[i]
                    );
                }
            },
            function (error) {
                alert(error.toString());
            });

You may notice that there are nested calls and I have added a path property to each participant object, expressing in an object model the fact that it is the path data for that particular participant. This also works fine.

Then I tried to add a method to the prototype like this:

var foo = participants[0];
foo.prototype.redraw = function(map) {
   ... //code that redraws
};

To my great surprise, I got an exception claiming that prototype is null:

JavaScript runtime error: Unable to set property 'redraw' of undefined or null reference

What is going on here?

This question seems to address the problem but I cannot see how to apply this information to this situation, since I do not know how to refer to a constructor that is not defined in my code.

In case this is useful info, inspection of participants.constructor in the immediate window at run-time appears to indicate that the constructor was Array()

Should I write a constructor that copies the fields into an object and sets a prototype? If so is there a javascript equivalent to C# reflection so I don't have to rewrite this for every type of query result object? NOTE coma answered this part of the question in comments while I was updating the question.

Answer 1

In this particular case, you're probably better off just adding the function to the object itself, not trying to add it to the prototype.

But to answer your question "How can I set the prototype when I don't know the name of the constructor?":

You can't set the prototype of an existing object (not in an a standard way). The prototype is assigned at construction time and cannot (in a standard way) be changed. You can change the properties of an object's prototype, but not the object that is its prototype.

The prototype of an object is not available from a property on it called prototype. The prototype property relates to functions. It's the object that is assigned as the prototype of objects created via that function using a new expession (e.g., new Foo() assigns Foo.prototype as the prototype of the newly-created object).

You can get the prototype of an object in ES5 via Object.getPrototypeOf. In some pre-ES5 implementations, it's available via the non-standard __proto__ property. (And in some of those implementations, __proto__ can be written to, which is why I said you couldn't swap out the prototype object in a standard way — you can in that non-standard way in implementations that support it.)

So in an ES5 environment, you could extend the prototype of an object like this:

Object.getPrototypeOf(theObject).newstuff = "foo";

But beware that that changes the prototype, and other objects may well be using it! For example:

// A constructor function, and a prototype to assign to objects created with it
function Foo() {
}
Foo.prototype.hi = function() {
    console.log("Hi");
};

// Create two objects
var f1 = new Foo();
var f2 = new Foo();

// Add something to the prototype of `f1`:
Object.getPrototypeOf(f1).there = function() {
    console.log("there");
};

// Since `f1` and `f2` *share* the same prototype object,
// `f2` now has it too!
f2.there(); // "there"

Live Copy | Source

Answer 2

To add a method to an object instead of a constructor you simply add it directly instead of via the prototype:

var foo = participants[0];
foo.redraw = function(map) {
   ... //code that redraws
};