Cannot set instance prototype using prototype = in javascript

Question

I'm struggling to set the instance of a protype. I've got something like this:

function Course() {
    // Some stuff
}

Course.prototype.MyMethod = function() {
    // Do stuff
}

Now if I create a New Course(), all works fine, however, I'm getting the Courses from JSON. So what I wanted to do is:

data.forEach(function(courseFromJSON) {
    courseFromJSON.prototype = Course.prototype;
    courseFromJSON.prototype = new Course(); // Doesn't work either
});

But my courses never get the method MyMethod set to them. Neither is setting courseFromJSON.prototype to new Course(). I've been going through Douglas Crockford's video's, but can't seem to get it right. What am I doing wrong?

Thanks.

http://stackoverflow.com/questions/7015693/how-to-set-the-prototype-of-a-javascript-object-that-has-already-been-instantiat — Vandesh, Oct 14 '13 at 08:49
Thanks @Vandesh, I had read that, the code uses the .prototype = something, which is what I tried. But it also states, that it only reflects on the child instances. Question remains: how do I get my prototype methods working on stuff incoming from JSON? — Jochen van Wylick, Oct 14 '13 at 08:54
ok, so courseFromJSON.__proto__ = Course.prototype seems to work — Jochen van Wylick, Oct 14 '13 at 08:58

score 1 · Accepted Answer · answered Oct 14 '13 at 09:25

Your code wasn't working because you can't change the prototype of an already created object, or at least not via the prototype property.

Two alternatives exist:

Use the internal __proto__ property, but this is not recommended because it is a non-standard property.
Use the setPrototypeOf(Object, prototype) function, this is recommended and will be standardized in ES6.

Or even better, use the code snippet taken from the documentation link on setPrototypeOf():

Object.setPrototypeOf = Object.setPrototypeOf || function (obj, proto) {
    obj.__proto__ = proto;
    return obj; 
}

Thanks, especially for the recommendation on which option to use. — Jochen van Wylick, Oct 14 '13 at 09:52

Cannot set instance prototype using prototype = in javascript

1 Answers1