Question

I’ve been using javascript for a while, but have never learned the language past the basics. I am reading John Resig’s ‘Pro Javascript Techniques’ – I’m coming up with some questions, but I’m not finding the answers to them in the book or on google, etc.

John gives this example in his book:
Function #1

function User( name, age ){   this.name = name;   this.age = age; } // Add a new function to the object prototype User.prototype.getName = function(){   return this.name; }; User.prototype.getAge = function(){   return this.age; }; var user = new User( 'Bob', 44 ); console.log('User: ' + user.getName() + ', Age: ' + user.getAge()); 

I’m still learning about the prototype property, so I tried writing something similar:
Function #2

function User (name, age ) {   this.name = name;   this.age = age;   this.getName = function() {     return this.name;   };   this.getAge = function() {     return this.age;   }; } var user = new User( 'Bob', 44 ); console.log('User: ' + user.getName() + ', Age: ' + user.getAge()); 

It doesn’t use the prototype property to create the getName and getAge functions, but the output is the same as John’s example.

I took it one step further, and created this:
Function #3

var User = {   name: '',   age: 0,   setName: function(name) {     this.name = name;   },   setAge: function(age) {     this.age = age;   },   getName: function() {     return this.name;   },   getAge: function() {     return this.age;   } }; User.setName('Bob'); User.setAge(44); console.log('User: ' + User.getName() + ', Age: ' + User.getAge()); 

Again – it looks different than John’s example (and I had to add setter methods), but the output is the same.

Question #1 – what is the difference between the 3 functions? What is the advantage of the prototype property, and is Function #2 doing anything incorrectly, because it seems more straight forward to code #2 instead of #1 (although I’m sure #1 is doing it better seeing as John created it).

Question #2 – How could I modify function #3 to not use the setName and setAge methods, but still keep the {…} shorthand? Can the {…} shorthand have constructors?

Thanks in advance for helping me learn!

EDIT I think my 2nd question was a little confusing. I meant how could I use the {…} shorthand to create a User object, but then after I create the object, say something like:

var user = new User('Bob', 44); 

Just like in Function #1 – or is that not possible?

EDIT #2 Wow! Thanks everyone for the awesome answers. That really makes it a lot more clear to me. So if I understand correctly, the difference between #1 and #2 aren’t too much. If I only ever create one ‘User’ object – they probably aren’t different at all. But if my program creates many User objects, #1 would most likely be more efficient and use less memory since all objects will share the same functions.

I really appreciate all of the great answers – Thanks!

Answer 1

Every time a function() {} is evaluated, it creates a new function object. Therefore, in #1 all of the User objects are sharing the same getName and getAge functions, but in #2 and #3, each object has its own copy of getName and getAge. All of the different getName functions all behave exactly the same, so you can’t see any difference in the output.

The {…} shorthand is a constructor. When evaluated, it constructs a new ‘Object’ with the given properties. When you run ‘new User(…)’, it constructs a new ‘User’. You happen to have created an Object with the same behavior as a User, but they are of different types.

Response to comment:

You can’t, directly. You could make a function that creates a new object as per #3. For example:

function make_user(name, age) {     return {         name: name,         age: age,         getName: function() { return name; },         getAge: function() { return age; },     }; }  var user = make_user('Joe', '18');