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dynamic-languages javascript prototype-oriented

How does JavaScript .prototype work?

2150

I’m not that into dynamic programming languages but I’ve written my fair share of JavaScript code. I never really got my head around this prototype-based programming, does any one know how this works?

var obj = new Object();
obj.prototype.test = function() { alert('Hello?'); };
var obj2 = new obj();
obj2.test();

I remember a lot discussion I had with people a while back (I’m not exactly sure what I’m doing) but as I understand it, there’s no concept of a class. It’s just an object, and instances of those objects are clones of the original, right?

But what is the exact purpose of this “.prototype” property in JavaScript? How does it relate to instantiating objects?

Update: correct way

var obj = new Object(); // not a functional object
obj.prototype.test = function() { alert('Hello?'); }; // this is wrong!

function MyObject() {} // a first class functional object
MyObject.prototype.test = function() { alert('OK'); } // OK

Also these slides really helped a lot.

10

  • 83

    John Resig has a few slides on function prototypes that were helpful to me when looking into the subject (you can also make changes to the code and see what happens…) http://ejohn.org/apps/learn/#64

    Feb 21, 2009 at 12:56

  • 5

    Great reference material, for purposes of keeping this question informative perhaps place some of the comments from John’s site on your answer in case his site is changes in a way that your link is no longer available. Either way +1, helped me.

    – Chris

    May 27, 2011 at 23:14


  • 96

    +1 for your link to John Resig’s JavaScript Ninja slide #64. Starting from there was really helpful, and I feel like I understand prototypes correctly.

    Jun 24, 2012 at 21:02

  • 4

    Do we really need a functional object for applying prototype? if yes than why ?

    – Anshul

    Feb 19, 2013 at 15:40

  • 6

    This might help you: webdeveasy.com/javascript-prototype

    – Naor

    Apr 10, 2013 at 11:35

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Every JavaScript object has an internal “slot” called [[Prototype]] whose value is either null or an object. You can think of a slot as a property on an object, internal to the JavaScript engine, hidden from the code you write. The square brackets around [[Prototype]] are deliberate, and are an ECMAScript specification convention to denote internal slots.

The value pointed at by the [[Prototype]] of an object, is colloquially known as “the prototype of that object.”

If you access a property via the dot (obj.propName) or bracket (obj['propName']) notation, and the object does not directly have such a property (ie. an own property, checkable via obj.hasOwnProperty('propName')), the runtime looks for a property with that name on the object referenced by the [[Prototype]] instead. If the [[Prototype]] also does not have such a property, its [[Prototype]] is checked in turn, and so on. In this way, the original object’s prototype chain is walked until a match is found, or its end is reached. At the top of the prototype chain is the null value.

Modern JavaScript implementations allow read and/or write access to the [[Prototype]] in the following ways:

  1. The new operator (configures the prototype chain on the default object returned from a constructor function),
  2. The extends keyword (configures the prototype chain when using the class syntax),
  3. Object.create will set the supplied argument as the [[Prototype]] of the resulting object,
  4. Object.getPrototypeOf and Object.setPrototypeOf (get/set the [[Prototype]] after object creation), and
  5. The standardized accessor (ie. getter/setter) property named __proto__ (similar to 4.)

Object.getPrototypeOf and Object.setPrototypeOf are preferred over __proto__, in part because the behavior of o.__proto__ is unusual when an object has a prototype of null.

An object’s [[Prototype]] is initially set during object creation.

If you create a new object via new Func(), the object’s [[Prototype]] will, by default, be set to the object referenced by Func.prototype.

Note that, therefore, all classes, and all functions that can be used with the new operator, have a property named .prototype in addition to their own [[Prototype]] internal slot. This dual use of the word “prototype” is the source of endless confusion amongst newcomers to the language.

Using new with constructor functions allows us to simulate classical inheritance in JavaScript; although JavaScript’s inheritance system is – as we have seen – prototypical, and not class-based.

Prior to the introduction of class syntax to JavaScript, constructor functions were the only way to simulate classes. We can think of properties of the object referenced by the constructor function’s .prototype property as shared members; ie. members which are the same for each instance. In class-based systems, methods are implemented the same way for each instance, so methods are conceptually added to the .prototype property; an object’s fields, however, are instance-specific and are therefore added to the object itself during construction.

Without the class syntax, developers had to manually configure the prototype chain to achieve similar functionality to classical inheritance. This led to a preponderance of different ways to achieve this.

Here’s one way:

function Child() {}
function Parent() {}
Parent.prototype.inheritedMethod = function () { return 'this is inherited' }

function inherit(child, parent) {
  child.prototype = Object.create(parent.prototype)
  child.prototype.constructor = child
  return child;
}

Child = inherit(Child, Parent)
const o = new Child
console.log(o.inheritedMethod()) // 'this is inherited'

…and here’s another way:

function Child() {}
function Parent() {}
Parent.prototype.inheritedMethod = function () { return 'this is inherited' }

function inherit(child, parent) {
    function tmp() {}
    tmp.prototype = parent.prototype
    const proto = new tmp()
    proto.constructor = child
    child.prototype = proto
    return child
}

Child = inherit(Child, Parent)
const o = new Child
console.log(o.inheritedMethod()) // 'this is inherited'

The class syntax introduced in ES2015 simplifies things, by providing extends as the “one true way” to configure the prototype chain in order to simulate classical inheritance in JavaScript.

So, similar to the code above, if you use the class syntax to create a new object like so:

class Parent { inheritedMethod() { return 'this is inherited' } }
class Child extends Parent {}

const o = new Child
console.log(o.inheritedMethod()) // 'this is inherited'

…the resulting object’s [[Prototype]] will be set to an instance of Parent, whose [[Prototype]], in turn, is Parent.prototype.

Finally, if you create a new object via Object.create(foo), the resulting object’s [[Prototype]] will be set to foo.

0

    1055

    Every JavaScript object has an internal “slot” called [[Prototype]] whose value is either null or an object. You can think of a slot as a property on an object, internal to the JavaScript engine, hidden from the code you write. The square brackets around [[Prototype]] are deliberate, and are an ECMAScript specification convention to denote internal slots.

    The value pointed at by the [[Prototype]] of an object, is colloquially known as “the prototype of that object.”

    If you access a property via the dot (obj.propName) or bracket (obj['propName']) notation, and the object does not directly have such a property (ie. an own property, checkable via obj.hasOwnProperty('propName')), the runtime looks for a property with that name on the object referenced by the [[Prototype]] instead. If the [[Prototype]] also does not have such a property, its [[Prototype]] is checked in turn, and so on. In this way, the original object’s prototype chain is walked until a match is found, or its end is reached. At the top of the prototype chain is the null value.

    Modern JavaScript implementations allow read and/or write access to the [[Prototype]] in the following ways:

    1. The new operator (configures the prototype chain on the default object returned from a constructor function),
    2. The extends keyword (configures the prototype chain when using the class syntax),
    3. Object.create will set the supplied argument as the [[Prototype]] of the resulting object,
    4. Object.getPrototypeOf and Object.setPrototypeOf (get/set the [[Prototype]] after object creation), and
    5. The standardized accessor (ie. getter/setter) property named __proto__ (similar to 4.)

    Object.getPrototypeOf and Object.setPrototypeOf are preferred over __proto__, in part because the behavior of o.__proto__ is unusual when an object has a prototype of null.

    An object’s [[Prototype]] is initially set during object creation.

    If you create a new object via new Func(), the object’s [[Prototype]] will, by default, be set to the object referenced by Func.prototype.

    Note that, therefore, all classes, and all functions that can be used with the new operator, have a property named .prototype in addition to their own [[Prototype]] internal slot. This dual use of the word “prototype” is the source of endless confusion amongst newcomers to the language.

    Using new with constructor functions allows us to simulate classical inheritance in JavaScript; although JavaScript’s inheritance system is – as we have seen – prototypical, and not class-based.

    Prior to the introduction of class syntax to JavaScript, constructor functions were the only way to simulate classes. We can think of properties of the object referenced by the constructor function’s .prototype property as shared members; ie. members which are the same for each instance. In class-based systems, methods are implemented the same way for each instance, so methods are conceptually added to the .prototype property; an object’s fields, however, are instance-specific and are therefore added to the object itself during construction.

    Without the class syntax, developers had to manually configure the prototype chain to achieve similar functionality to classical inheritance. This led to a preponderance of different ways to achieve this.

    Here’s one way:

    function Child() {}
    function Parent() {}
    Parent.prototype.inheritedMethod = function () { return 'this is inherited' }
    
    function inherit(child, parent) {
      child.prototype = Object.create(parent.prototype)
      child.prototype.constructor = child
      return child;
    }
    
    Child = inherit(Child, Parent)
    const o = new Child
    console.log(o.inheritedMethod()) // 'this is inherited'
    

    …and here’s another way:

    function Child() {}
    function Parent() {}
    Parent.prototype.inheritedMethod = function () { return 'this is inherited' }
    
    function inherit(child, parent) {
        function tmp() {}
        tmp.prototype = parent.prototype
        const proto = new tmp()
        proto.constructor = child
        child.prototype = proto
        return child
    }
    
    Child = inherit(Child, Parent)
    const o = new Child
    console.log(o.inheritedMethod()) // 'this is inherited'
    

    The class syntax introduced in ES2015 simplifies things, by providing extends as the “one true way” to configure the prototype chain in order to simulate classical inheritance in JavaScript.

    So, similar to the code above, if you use the class syntax to create a new object like so:

    class Parent { inheritedMethod() { return 'this is inherited' } }
    class Child extends Parent {}
    
    const o = new Child
    console.log(o.inheritedMethod()) // 'this is inherited'
    

    …the resulting object’s [[Prototype]] will be set to an instance of Parent, whose [[Prototype]], in turn, is Parent.prototype.

    Finally, if you create a new object via Object.create(foo), the resulting object’s [[Prototype]] will be set to foo.

    0

      196

      This is a very simple prototype based object model that would be considered as a sample during the explanation, with no comment yet:

      function Person(name){
          this.name = name;
      }
      Person.prototype.getName = function(){
          console.log(this.name);
      }
      var person = new Person("George");
      

      There are some crucial points that we have to consider before going through the prototype concept.

      1- How JavaScript functions actually work:

      To take the first step we have to figure out, how JavaScript functions actually work , as a class like function using this keyword in it or just as a regular function with its arguments, what it does and what it returns.

      Let’s say we want to create a Person object model. but in this step I’m gonna be trying to do the same exact thing without using prototype and new keyword.

      So in this step functions, objects and this keyword, are all we have.

      The first question would be how this keyword could be useful without using new keyword.

      So to answer that let’s say we have an empty object, and two functions like:

      var person = {};
      function Person(name){  this.name = name;  }
      
      function getName(){
          console.log(this.name);
      }
      

      and now without using new keyword how we could use these functions. So JavaScript has 3 different ways to do that:

      a. first way is just to call the function as a regular function:

      Person("George");
      getName();//would print the "George" in the console
      

      in this case, this would be the current context object, which is usually is the global window object in the browser or GLOBAL in Node.js. It means we would have, window.name in browser or GLOBAL.name in Node.js, with “George” as its value.

      b. We can attach them to an object, as its properties

      The easiest way to do this is modifying the empty person object, like:

      person.Person = Person;
      person.getName = getName;
      

      this way we can call them like:

      person.Person("George");
      person.getName();// -->"George"
      

      and now the person object is like:

      Object {Person: function, getName: function, name: "George"}
      

      The other way to attach a property to an object is using the prototype of that object that can be find in any JavaScript object with the name of __proto__, and I have tried to explain it a bit on the summary part. So we could get the similar result by doing:

      person.__proto__.Person = Person;
      person.__proto__.getName = getName;
      

      But this way what we actually are doing is modifying the Object.prototype, because whenever we create a JavaScript object using literals ({ ... }), it gets created based on Object.prototype, which means it gets attached to the newly created object as an attribute named __proto__ , so if we change it, as we have done on our previous code snippet, all the JavaScript objects would get changed, not a good practice. So what could be the better practice now:

      person.__proto__ = {
          Person: Person,
          getName: getName
      };
      

      and now other objects are in peace, but it still doesn’t seem to be a good practice. So we have still one more solutions, but to use this solution we should get back to that line of code where person object got created (var person = {};) then change it like:

      var propertiesObject = {
          Person: Person,
          getName: getName
      };
      var person = Object.create(propertiesObject);
      

      what it does is creating a new JavaScript Object and attach the propertiesObject to the __proto__ attribute. So to make sure you can do:

      console.log(person.__proto__===propertiesObject); //true
      

      But the tricky point here is you have access to all the properties defined in __proto__ on the first level of the person object(read the summary part for more detail).


      as you see using any of these two way this would exactly point to the person object.

      c. JavaScript has another way to provide the function with this, which is using call or apply to invoke the function.

      The apply() method calls a function with a given this value and
      arguments provided as an array (or an array-like object).

      and

      The call() method calls a function with a given this value and
      arguments provided individually.

      this way which is my favorite, we can easily call our functions like:

      Person.call(person, "George");
      

      or

      //apply is more useful when params count is not fixed
      Person.apply(person, ["George"]);
      
      getName.call(person);   
      getName.apply(person);
      

      these 3 methods are the important initial steps to figure out the .prototype functionality.


      2- How does the new keyword work?

      this is the second step to understand the .prototype functionality.this is what I use to simulate the process:

      function Person(name){  this.name = name;  }
      my_person_prototype = { getName: function(){ console.log(this.name); } };
      

      in this part I’m gonna be trying to take all the steps which JavaScript takes, without using the new keyword and prototype, when you use new keyword. so when we do new Person("George"), Person function serves as a constructor, These are what JavaScript does, one by one:

      a. first of all it makes an empty object, basically an empty hash like:

      var newObject = {};
      

      b. the next step that JavaScript takes is to attach the all prototype objects to the newly created object

      we have my_person_prototype here similar to the prototype object.

      for(var key in my_person_prototype){
          newObject[key] = my_person_prototype[key];
      }
      

      It is not the way that JavaScript actually attaches the properties that are defined in the prototype. The actual way is related to the prototype chain concept.


      a. & b. Instead of these two steps you can have the exact same result by doing:

      var newObject = Object.create(my_person_prototype);
      //here you can check out the __proto__ attribute
      console.log(newObject.__proto__ === my_person_prototype); //true
      //and also check if you have access to your desired properties
      console.log(typeof newObject.getName);//"function"
      

      now we can call the getName function in our my_person_prototype:

      newObject.getName();
      

      c. then it gives that object to the constructor,

      we can do this with our sample like:

      Person.call(newObject, "George");
      

      or

      Person.apply(newObject, ["George"]);
      

      then the constructor can do whatever it wants, because this inside of that constructor is the object that was just created.

      now the end result before simulating the other steps:
      Object {name: “George”}


      Summary:

      Basically, when you use the new keyword on a function, you are calling on that and that function serves as a constructor, so when you say:

      new FunctionName()
      

      JavaScript internally makes an object, an empty hash and then it gives that object to the constructor, then the constructor can do whatever it wants, because this inside of that constructor is the object that was just created and then it gives you that object of course if you haven’t used the return statement in your function or if you’ve put a return undefined; at the end of your function body.

      So when JavaScript goes to look up a property on an object, the first thing it does, is it looks it up on that object. And then there is a secret property [[prototype]] which we usually have it like __proto__ and that property is what JavaScript looks at next. And when it looks through the __proto__, as far as it is again another JavaScript object, it has its own __proto__ attribute, it goes up and up until it gets to the point where the next __proto__ is null. The point is the only object in JavaScript that its __proto__ attribute is null is Object.prototype object:

      console.log(Object.prototype.__proto__===null);//true
      

      and that’s how inheritance works in JavaScript.

      The prototype chain

      In other words, when you have a prototype property on a function and you call a new on that, after JavaScript finishes looking at that newly created object for properties, it will go look at the function’s .prototype and also it is possible that this object has its own internal prototype. and so on.

      0