Question

I want to create an object. And, in almost code I have read, they often use this style:

function student(_id, _name, _year){
   this.id = _id;
   this.name = _name;
   this.year = _year;
}

But, I don’t know what the difference with below code :

function student (_id, _name, _year){
   var id = _id;
   var name = _name;
   var year = _year;
}

I have tested for example, alert properties to screen, and see no difference.

Thanks 🙂

Answer 1

When you declare variables using var they are only visible in the scope of your function/constructor. They are private so to say.

Using this, in this case, goes hand in hand with a constructor function. When you instantiate a student all the values assigned to this will be publicly accessible.

First I recommend renaming your student into Student with a capital S. This is a convention that indicates it is a constructor and you need to use the new keyword.

function Student(id, name, year){
   this.id = id;
   this.name = name;
   this.year = year;
}

If you now instantiate the Student you can access the values…

var student = new Student(1, "Name", 2012);
console.log(student.year); // => 2012

When using var you can’t…

function Student (id, name, year){
   var id = id;
   var name = name;
   var year = year;
}

var student = new Student(1, "Name", 2012);
console.log(student.year); // => undefined