Assignment operator for functions

Question

I'm having a bit of trouble understanding what the assignment operator is used for when dealing with methods, functions, etc. Here is the example in w3 school for defining an object

function person(firstname,lastname,age,eyecolor){
this.firstname=firstname;
this.eyecolor=eyecolor;

this.newlastname=newlastname;
}

and this is the actual function (place somewhere else)

function newlastname(new_lastname){
this.lastname=new_lastname;
}

It's just very weird to me throughout javascript, you say

object.methodname = somefunctionname

Any ideas to help me conceptualize it?

James Allardice · Accepted Answer · 2012-06-19 07:34:12Z

2

The code in your question is effectively the same as this:

function person(firstname, lastname, age, eyecolor) {
    this.firstname = firstname;
    this.eyecolor = eyecolor;

    //anonymous function assigned to newlastname property
    this.newlastname = function(new_lastname) {
        this.lastname = new_lastname;
    };
}

person is a constructor function (you would call it with the new operator to create a new instance). Every instance of person has three properties, firstname, eyecolor and newlastname.

The newlastname property is a method since it's value is a function. When you call that method, the instance of person on which it is called will get a lastname property.

For example:

var me = new person("James", "Allardice", 22, "Brown");
me.lastname; //undefined
me.newlastname("Something");
me.lastname; //Something

This is possible because in JavaScript, functions are objects.

answered Jun 19, 2012 at 7:34

James Allardice

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2 Comments

Ben Over a year ago

properties are methods...? I'm so mind blown right now. Properties can have functions as values...what is this madness!!! (coming from c++/c#/objective-c)

James Allardice Over a year ago

Yeah, I think that's a common reaction from developers more used to C/C++! The article lanzz linked to should help explain the general concept in more detail.

lanzz · Accepted Answer · 2012-06-19 07:35:06Z

1

This language feature is called First-class functions. The Wikipedia article is pretty comprehensive.

answered Jun 19, 2012 at 7:35

lanzz

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Comments

Christoph · Accepted Answer · 2012-06-19 07:42:51Z

1

This is the cool thing about javascript. Functions are first-class objects, this means unlike other non-functional programming languages, you can hand them as parameters to other functions, return them from function and (like in your example) attach them to objects like a normal property.

This enables programming paradigmas like the (for the web) so important asynchronous function calls (callbacks).

answered Jun 19, 2012 at 7:42

Christoph

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