Basic JavaScript Part 9: Reusing Methods of Other Objects

Here are the links to the previous installments:

1. Functions
2. Objects
3. Prototypes
4. Enforcing New on Constructor Functions
5. Hoisting
6. Automatic Semicolon Insertion
7. Static Properties and Methods
8. Namespaces

In one of my previous posts, I mentioned that functions in JavaScript are plain objects that can have their own properties and methods. One of these methods that are available for every function object is a method named call(), which is defined on the prototype of Function. This method allows you to ?reuse? a method from another object. Let?s talk code.Suppose we have an object called podcast which has a download() function:

var podcast = {
    name : 'Astronomy podcast',
    download : function(episode) {
        console.log('Downloading ' + episode + ' of ' + this.name);
    }
};

podcast.download('the first episode');

The output we get by executing this function is exactly what we expect it to be:

?Downloading the first episode of Astronomy podcast?

Now suppose we bring on another object called screencast :

var screencast = {
    name: 'Node tuts'
};

Wouldn?t it be nice if we could somehow reuse the download() method of the podcast object so that we can download a screencast as well. We can do this by invoking call() on the download() method like so: 

podcast.download.call(screencast, 'the last episode');

This gives us the following output:

?Downloading the last episode of Node tuts?

What exactly happened here? Well, we just invoked the call() method on the download function object, specifying a reference to the screencast object and a string argument that describes the particular episode. The download() method got invoked with the screencast object bound to this. Therefore, the download() method uses the name property of the screencast object instead of the name of the podcast object.

There?s also another method with similar functionality defined on the Function prototype named apply(). This method behaves exactly the same as the call() method, except that the arguments passed to the underlying method in question need to be passed as an array.

podcast.download.apply(screencast, ['the last episode']);

Utilizing the call()/apply() methods is a very powerful way to reuse features of objects that are built into JavaScript (like Array, String, etc. ?) or from third-party libraries. But I don?t recommend applying this technique when you own the code of the method you want to reuse. A simple refactoring like extracting the particular method into another object is a much better approach.

Until next time.