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javascript tutorial

1. understanding prototype chain.

Everything is object in javascript, including type object and instance object, type object defines a type, which can be instantiated and create new instance object of that type, instance object is created from the corresponding type object.

function ggg(){
   this.name='hhh'
}
ggg.prototype.xxx=true;
ggg.prototype.kkk=0;

function or class declaration declares prototype definition of a NEW type object, the above code is equivalent to ggg.prototype.constructor=ggg, the Symbol ggg only represents a type object, It is a function or constructor object which is used to construct an instance object of the declared type, so all type objects are function or constructor.

1.1 Inheritance hierarchy of type object.

The root of all type object is the object Object.prototype, all type object originate from it, also it is true for instance object.

  • String object --> function () --> Object.prototype --> null
  • Number object --> function () --> Object.prototype --> null
  • Object object --> function () --> Object.prototype --> null
  • Array object --> function () --> Object.prototype --> null
  • Boolean object --> function () --> Object.prototype --> null
  • ...

As we can see, most of type objects have a type of function ()

For a type object, we have the followings:

  • Object.prototype indicates an object whose attributes will be inherited by all instance objects of the type.
  • Object.__proto__ indicates Object's prototype, where attributes of Object come from, this also applies for an instance object, but an instance object doesn't have the attribute prototype which is dedicated to type object.

1.2 Inheritance hierarchy of instance object.

An instance object contains its own properties and inherits properties from the property TypeName.prototype of its type, it means the object indicated by the property TypeName.prototype of a type object will be inherited by any instance of that type, the instance object has a property proto which holds a reference to the object TypeName.prototype, all instances of that type share the inherited object.

var a=new String();
// a --> String.prototype --> Object.prototype --> null;
function Foo(){this.name="hellp";this.age=20;}
var test=new Foo()
//test --> Foo.prototype --> Object.prototype --> null;

You can also define attributes on instance object with the following methods:

  1. Those defined inside a constructor function that are given to object instances. These are fairly easy to spot — in your own custom code, they are the members defined inside a constructor using the this.x = x type lines; in built-in browser code, they are the members only available to object instances (usually created by calling a constructor using the new keyword, e.g. let myInstance = new myConstructor()).
  2. Those defined directly on the constructor themselves, that are available only on the constructor. These are commonly only available on built-in browser objects, and are recognized by being chained directly onto a constructor, not an instance. For example, Object.keys(). These are also known as static properties/methods.
  3. Those defined on a constructor's prototype, which are inherited by all instances and inheriting object classes. These include any member defined on a Constructor's prototype property, e.g. myConstructor.prototype.x().
  4. Those available on an object instance, which can either be an object created when a constructor is instantiated like we saw above (so for example var teacher1 = new Teacher( name = 'Chris' ); and then teacher1.name), or an object literal (let teacher1 = { name = 'Chris' } and then teacher1.name).