JavaScript Inheritance: "Template Method" Pattern and overriding functions

Question

Have a look at this snipped:

There is a transformator with a transformMap()-method. This method calls two other methods of the transformator: transformMapHead() and transformMapBody()

transformator

var transformator =
{
    transformMap: function(tree) {
        return transformator.transformMapHead(tree)
              +transformator.transformMapBody(tree.body[0].body) +"}\n";
    },  

    transformMapHead: function(tree) { 
        return "to be done"; 
    },

    transformMapBody: function(tree) { 
        // completly implemented
    },  
    ...
};

Ok, fine so far. The transformMapBody()-method is completly implemented, the transformMapHead()-method isn't, this will be done in a child-object.

Now, lets have a look at a more concrete transformator. Here I only want to implement the transformMapHead()-method.

But, when I implement the

concreteTransformator

var concreteTransformator = Object.beget(transformator);                            

concreteTransformator.transformMapHead = function(tree)
{
    // my business logic
    console.log("i am the new business logic");
};

it doesn't work correctly. Instead of running the transformMapHead()-method from the concreteTransformator-object, the method from the first transformator-object is executed and "to be done" ist printed out.

I can avoid this by copying the transformMap()-method from the transformer-object to the concreteTransformer-object:

concreteTransformator.transformMap =  function(tree) {
    return concreteTransformator.transformMapHead(tree)
          +concreteTransformator.transformMapBody(tree.body[0].body) +"}\n";
};

Then, everything runs fine, but this cannot be the way to go?!

At last, the beget-helper-function from Douglas Crockford:

if (typeof Object.beget !== 'function') {
Object.beget = function(o) {
    var F = function() {};
    F.prototype = o;
    return new F();
};

}

Felix Kling · Accepted Answer · 2012-09-29 20:45:06Z

1

The problem is that you are referencing a specific variable inside transformMap (transformator), which contains a reference to the original object. To understand the implications, consider what would happen if you did:

var foo = transformator;
transformator = null;
foo.transformMap();

You would get an error because transformator is null now and therefore does not have a property transformMapHead anymore.

You should be using this instead:

return this.transformMapHead(tree)
       +this.transformMapBody(tree.body[0].body) +"}\n";

to refer to the current object.

Learn more about this.

answered Sep 29, 2012 at 20:45

Felix Kling

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3 Comments

Cornelius Schmale Over a year ago

Hi Felix, thanks for your answer. I tried 'this' before. Using "this" results in a TypeError: Object #<Object> has no method 'transformMapHead'

Felix Kling Over a year ago

How are you calling transformMap? It must be concreteTransformator.transformMap(). There are other ways to explicitly set what this should refer to inside a function, but we would have to know how you call the function in order to give a proper suggestion.

Cornelius Schmale Over a year ago

Mea Culpa! The problem was in the calling-statement, inside the nodejs-exports-beget-aliencode :) Thanks a lot!

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