为什么构造函数不能访问他原型的属性，但可以访问父原型的属性（原型的原型）

Question

Following code, i checked using chrome browser console :

function A(){
  this.a='a'
}

this is a constructor function.i have assined a property b to the prototype of A .

A.prototype.b='b';

but when i access b property using constructor function A . it is showing undefined .

A.b //it return undefined

again i assigned property m to the Function.prototype , and the access using A it return coorect vaue.

Function.prototype.m='m';
A.m //it return "m"

So why constructor function can't access his prototype property,but can access parent prototype (protype of his prototype) property .

Answer 1

You can't access A.prototype.b because you haven't instantiated A . Prototypes are objects which are created during the instantiation of an Object . In your example, here's how this might look:

// Create your constructor and prototype object
function A() {}
A.prototype = {
  b: "b"
};

var a = new A();
console.log(a.b); 
>>> "b"

However, in the case of Am , A (note, capital A, your constructor), is an instance of Function , and so inherits all of it's attributes, including the one you've added.

For more, I suggest you look at the MDN article on an introduction to Object-Oriented JavaScript

Answer 2

You cannot access it as Ab since A is not an instance of A constructor. You need to:

var instanceA = new A();
instanceA.b; // will work

Whereas

Function.prototype.m='m';
A.m //it return "m"

does work because A is an instance of the Function object.