Get the properties of the function object

Question

I am learning object oriented javascript, from Object Palyground

according to the tutorial, function itself is considered as an object and i know that you can get the properties of an object using in keyword in javascript

var obj = {a:"hello",b=123};
for(var prop in obj)
alert(var);

The above code gives me the keys present in the function object obj as expected

but when I use the same code for this situation

var funObj = function(a,b){
   alert("hello");
}
for(var prop in funObj)
alert(prop);

or in this case even

function myFunction(a,b){
 alert("hello");
}
for(var prop in myFunction)
alert(prop);

It does not give me any output, Ideally according to the tutorial the function object should comprise of three properties that are: name, length and prototype but i am getting none, where i am doing wrong

kindly help me

score 4 · Accepted Answer · answered Jul 16 '15 at 17:38

4

where i am doing wrong

for...in only iterates over enumerable properties. The default properties of native objects are often non-enumerable.

You could use Object.getOwnPropertyNames to get a list of properties of the function object:

> Object.getOwnPropertyNames(function() {})
["length", "name", "arguments", "caller", "prototype"]

Or if you just want to inspect the properties for learning purposes, use console.dir to log the function:

enter image description here

answered Jul 16 '15 at 17:38

Felix Kling

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Thanks a lot that was what i was looking for – Siddarth Jul 16 '15 at 18:01
One more query, If i understand correctly when you say ENUMERABLE properties does that mean that the user defined ones instead of native ones? – Siddarth Jul 16 '15 at 18:02
Not necessarily. By default, properties created through assignment, e.g. `foo.bar = 42;` are enumerable. However, you can also create non-enumerable properties via `Object.defineProperty(foo, 'bar', {value: 42, enumerable: false});`. So, "enumerability" really is a characteristic of every property. – Felix Kling Jul 16 '15 at 18:04
Thanks again, I am from Java background so these concepts are little confusing to me ;) – Siddarth Jul 16 '15 at 18:19

score 0 · Answer 2 · answered Jul 16 '15 at 17:46

var obj = {a:"hello",b=123};

if not the same as

var obj = function(a, b) {}

If you want A and B to be properties, you should do it that way:

var obj = function(a, b) {
   this.a = a;
   this.b = b;
}

That way, obj.a and obj.b make a little more sense, but not that much yet. function(){} is you constructor. So you need to actually build an instance in order to use it the right way.

var myObj = new obj('value for a', 'value for b');
for (var prop in myObj) {
   // Prop = a || b
   console.log(prop);

   // Values for a and b
   console.log( myObj[ prop ] );
}

If you use it in a static way, you do NOT have to do the function thing. Also not that object don't have the length method. Arrays do.

To make this REALLY SIMPLE, and totally not accurate, {} (objects) are kinda static and function(){} are instanciable (does that word even exists?), which means you can use the "new" and therefore benefit from the function constructor.

This is all really technic, and it looks like you're beginning with javascript. Tell me if I was unclear or you need some precision.

Hope that helps.

Hi Bene, Thanks for the quick response, I am good in Java and not that comfortable with javascript i am trying to learn the object oriented concepts, According to the javascript documentation — Siddarth, Jul 16 '15 at 17:53

score 0 · Answer 3 · edited May 23 '17 at 11:58

A Function in JavaScript can be referred to as "first class" as it is a callable object meaning it is an object with callable semantics added to it. In some ways an Array in JavaScript is similar to a Function in that they are both objects but have special features included. NOTE: A common statement uttered about JavaScript is that "everything is an object" but this is not true however. None of the simple primitives (string, boolean, number, null, and undefined) are objects (although JS does some nifty tricks if you attempt to treat them as such).

var obj = {bar: 'hi'};
Object.keys(obj); // ["bar"]

function func() {}
Object.keys(func); // []
func.bar = 'hello';
Object.keys(func); // ["bar"];

var arr = [1, 2, 3];
Object.keys(arr); // ["0", "1", "2"]
arr.bar = 'hey';
Object.keys(arr); // ["0", "1", "2", "bar"]

bar shows up in all three because those properties are set to be enumerable by default. However as you noted a function has some other properties but those aren't showing up. Why? Let's look at obj first.

obj.propertyIsEnumerable('bar'); // true

bar by default was set to be enumerable.

Now lets look at bar in func.

func.propertyIsEnumerable('bar'); // true

So that explains why bar shows up for func but what about name?

func.propertyIsEnumerable('name'); // false

Ahah! It is not enumerable. This is true for a lot of properties/methods that are automatically linked to or assigned to an object via the JavaScript engine. For instance, the length property of an array object.

arr.propertyIsEnumerable('length'); // false

That all said, if you are new to JS I would highly recommend reading the You Don't Know JS series by Kyle Simpson. It's meant for people with some programming experience but they are very useful for learning what JS is and what it's not (no matter how much syntactic sugar gets piled on). I would especially recommend focusing on the key point of the difference between classical inheritance and JavaScript's OLOO (Objects Linking to Other Objects).

The intro book Up & Going is a quick read that covers the basics of what the entire series will dive deeper into. The rest of the series (so far) includes: Types & Grammar, Scopes & Closures, this & Object Prototypes, Async & Performance, and ES6 & Beyond. You can also preview them all on his github repository.

OR You can also review MDN's documentation for Working with Objects for some very good information.

Get the properties of the function object

3 Answers3