over 1 year ago

Originally, I thought there is only one prototype property per function until I read about __proto__. Take a look here:

function Person(){
  this.name = 'John';
}

Person.prototype // Object{}

                 //    > constructor: function Person()

                 //    > __proto__: Object

          
Person.__proto__ // function() {}

I was totally confused until I read this stackoverflow post. First of all, this graph helps to clarify things a bit:

The accept answer nails it:

__proto__ is the actual object that is used in the lookup chain to resolve methods, etc. prototype is the object that is used to build __proto__ when you create an object with new:

( new Foo ).__proto__ === Foo.prototype
( new Foo ).prototype === undefined

As one of the users commented:

Ah! So prototype is not available on the instances themselves, but only on the constructor functions.

Here is a table to summarize the idea, considering the code blow:

var obj = {}

function Person(){
}

var p1 = new Person()
__proto__ prototype
obj Object undefined
Person() Function Object {
  constructor: function Person()
  __proto__: Object
}
p1 Object {
  constructor: function Person()
  __proto__: Object
}
undefined

Note: obj.__proto__ is the Object.prototype and Person._proto__ is Function.prototype

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