Javascript prototype won't update a method

Question

I'm learning to use javascript's prototype feature (not the library). I thought I could replace an object's method by using MyObject.prototype.myFunction = function () { ... }. Apparently this doesn't work.

The code below defines an object, and replaces its function using prototype. Run it in your browser, the console still shows the original output.

    <script type="text/javascript">
        function TestObject() {
            this.testFunction = function() {
                console.log("Original function output");
            }
        }

        // This should replace the method defined in the object.
        TestObject.prototype.testFunction = function() {
            console.log("YOU GOT CHANGED");
        }

        var HelloWorld = new TestObject();

        HellowWorld.testFunction(); // Should output "YOU GOT CHANGED"
    </script>

The instance method shadows any prototype method.

Paul S.
– Paul S.

2013-04-08 17:22:31 +00:00
Commented Apr 8, 2013 at 17:22 — Paul S.
– Paul S., Commented Apr 8, 2013 at 17:22

Paul S. · Accepted Answer · 2013-04-08 17:29:39Z

2

Instance methods shadow methods inherited through the prototype chain

var HelloWorld = new TestObject();
HelloWorld.testFunction(); // finds method on instance
// "Original function output"
delete HelloWorld.testFunction; // delete method from instance
HelloWorld.testFunction(); // not found on instance, look in prototype, found
// "YOU GOT CHANGED"

answered Apr 8, 2013 at 17:29

Paul S.

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Comments

Loamhoof · Accepted Answer · 2013-04-08 17:23:49Z

1

Not really, in your constructor, you're overriding the prototype. The original code is the prototype's one (it's bound as soon as you create the object, before running it).

answered Apr 8, 2013 at 17:23

Loamhoof

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1 Comment

Don P Over a year ago

When you say "it's bound", what are you referring to?

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