javascript - When I assign prototype to function I get undesired output

Question

Please follow the code below,

var fn79 = function(){

    var Student = function(_name){
        this.name = _name;
    };


    Student.prototype = function(){
        print("Inside Prototype function");
    };


    //Student.prototype = {}

    var obj1 = new Student("Ricky");

    Student.prototype.lastName = "Gonzales";

    var obj2 = new Student("Jacky");

    print(obj1.name+" - "+obj1.lastName);
    print(obj2.name+" - "+obj2.lastName);   
};

fn79();

The output I get is

D:Rahul ShivsharanMyPractiseJavaScriptCommandLine>java -jar js-14.jar prac.js
- Gonzales
- Gonzales

D:Rahul ShivsharanMyPractiseJavaScriptCommandLine>

From above output you can see I am not able to print "name" property of objects.

now if I change the the code to as below,

    var fn79 = function(){

    var Student = function(_name){
        this.name = _name;
    };

    /*
    Student.prototype = function(){
        print("Inside Prototype function");
    };
    */

    Student.prototype = {}

    var obj1 = new Student("Ricky");

    Student.prototype.lastName = "Gonzales";

    var obj2 = new Student("Jacky");

    print(obj1.name+" - "+obj1.lastName);
    print(obj2.name+" - "+obj2.lastName);   
};

fn79();

I get the desired output as

D:Rahul ShivsharanMyPractiseJavaScriptCommandLine>java -jar js-14.jar prac.js
Ricky - Gonzales
Jacky - Gonzales

D:Rahul ShivsharanMyPractiseJavaScriptCommandLine>

Why my First example was not working properly.

Function is an object itself in javascript.

What I thought is,

Student.prototype = function(){
       print("Inside Prototype function");
}

Student's prototype is pointing to function which itself is an object.

So why "name" is not getting printed in my first case, and how prototype assigning to function effects it.

Answer 1

The problem with functions is that they do have a non-writable .name property. This non-writability even affects the objects that inherit from the function, in your case the Student instances, so the assignment fails:

var Student = function(name){
    console.log(this.name);
    this.name = name;
    console.log(this.name);
};
Student.prototype = function ProtoName() {};
console.log(Student.prototype.name);
new Student("test");

You'll just see the string "ProtoName" thrice.

You can use strict mode to make it more obvious that the .name = fails:

var Student = function(name){
    "use strict";
    this.name = name;
};
Student.prototype = function ProtoName() {};
new Student("test");

You'll get an Error: Invalid assignment in strict mode.

You can work around the non-writability by creating the .name property on the Student instance using Object.defineProperty instead of the simple assignment, but really you just should not use a function object as a prototype.