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JS101: Prototypes

Alex R. Young

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tutorials language js101 beginner

JS101: Prototypes

Posted by Alex R. Young on .
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tutorials language js101 beginner

JS101: Prototypes

Posted by Alex R. Young on .
JS101 is a tutorial series aimed at beginners. Each post is a bite-sized chunk aimed to elucidate JavaScript fundamentals. To read previous posts, view the js101 tag.

After spending years studying object oriented programming, adapting to JavaScript can be frustrating. In particular, the lack of a class keyword is a source of confusion. However, JavaScript's design needn't be a hindrance -- mastering its prototype-based inheritance will improve your understanding of the language.

The first thing to realise is there should be a distinction between object-oriented programming and class-oriented. JavaScript gives us the tools we need to do most of the things languages with classes can do -- we just need to learn how to use it properly.

Let's take a brief look at the prototype property to see how it can deepen our knowledge of JavaScript.

The prototype Property

The prototype property is an internal property, and it's designed to be used to implement inheritance. What we mean by "inheritance" here is a specific form of inheritance. Because both state and methods are carried by objects, then we can say that structure, behaviour, and state are all inherited (ES5: Objects). This is in contrast to class-based languages, where state is carried by instances and methods are carried by classes.

A constructor is a function that has a property named prototype:

function Animal() {  
}

console.log(Animal.prototype);  

This displays {} -- the Animal object has a prototype property, but there's nothing user-defined in it yet. We're free to add values and methods as we please:

function Animal() {  
}

Animal.prototype.type = 'Unknown';  
Animal.prototype.weight = 0;  
Animal.prototype.weightUnits = 'kg';

Animal.prototype.toString = function() {  
  return this.type + ', ' + this.weight + this.weightUnits;
};

var molly = new Animal();  
molly.type = 'Dog';  
molly.weight = 28;

console.log(molly.toString());  

This would display "Dog, 28kg". We can group these assignments together by using an object literal:

function Animal() {  
}

Animal.prototype = {  
  type: 'Unknown',
  weight: 0,
  weightUnits: 'kg',

  toString: function() {
    return this.type + ', ' + this.weight + this.weightUnits;
  }
};

This shouldn't look too different from the classes you might be more familiar with.

Dynamic Prototypes

Properties can be added to objects dynamically, simply by assigning values:

var molly = new Animal()  
  , harley = new Animal();

molly.type = 'Dog';  
molly.weight = 28;

harley.type = 'Dog';  
harley.weight = 38;  
harley.name = 'Harley';

console.log(molly);  
console.log(harley);

// { type: 'Dog', weight: 28 }
// { type: 'Dog', weight: 38, name: 'Harley' }

Adding the name property here only affects that instance. However, the constructor's prototype can be changed, and this will affect objects made with that prototype:

Animal.prototype.weightUnits = 'oz';

console.log(molly.toString())  
// Now displays 'Dog, 28oz'

This is why people boast that their libraries don't touch native prototypes, or only do so safely -- it's entirely possible to change the expected built-in functionality of objects like String to do unsafe things:

String.prototype.match = function() {  
  return true;
};

console.log('alex'.match(/1234/));  

This returns true, so I've now succeeded in breaking a fundamental method that many JavaScript programs rely on.

Modifying native prototypes isn't always bad; people use it for useful things like patching support for more modern versions of ECMAScript in older browsers.

What happens if we replace the prototype property?

var molly = new Animal()  
  , harley;

molly.type = 'Dog';  
molly.weight = 28;

Animal.prototype = {  
  toString: function() {
    return '...';
  }
};

harley = new Animal;  
harley.type = 'Dog';  
harley.weight = 38;

console.log(molly.toString());  
console.log(harley.toString());

// Dog, 28kg
// ...

Despite the fact changing a prototype affects all of the instances, replacing a constructor's prototype entirely does not affect older instances. Why? Well, instances have a reference to the prototype rather than a discrete copy. Think of it like this: each instance created by the new keyword is connected to the original prototype.

Next Week

In the spirit of keeping this post bite-sized, I'll finish here. Next week we'll look at prototype chains and inheritance in more detail.

References