Function not listed under Object.keys or Object.getOwnPropertyNames but can be invoked

Question

Suppose there is some library javascript object jsObj. On calling Object.keys or Object.getOwnPropertyNames , I get a list of properties like

[a,b,c,d]

But still I can call a function like jsObj.e(). Why the the method e is not part of Object.keys or Object.getOwnPropertyNames? How they are doing it?

Here, it says that Object.getOwnPropertyNames will return non enumerable properties too. So what is the characterstic of property like e above.

I am using opentok server side SDK. Using following code,

var OpenTok = require('opentok');
var opentok = new OpenTok(config.tokbox.apiKey, config.tokbox.secret);
console.log("opentok1", Object.getOwnPropertyNames(opentok));
prints -> // [ '_client',
  'apiKey',
  'apiSecret',
  'apiUrl',
  'startArchive',
  'stopArchive',
  'getArchive',
  'deleteArchive',
  'listArchives' ] 
console.log("opentok2", opentok.createSession);
prints -> function (...){...}

Do share executable demo.. Unable to understand the issue.... — Rayon, May 19 '16 at 10:13
It's probably inherited from one of its prototypes? If you do this: `for(prop in obj) { console.log(prop); }` (where obj is your object), does it get outputted then? — Arg0n, May 19 '16 at 10:15
Added some code, not executable because you need your own api keys. Hopefully, that will explain the question clearly — Aman Gupta, May 19 '16 at 10:30

Emil Ingerslev · Accepted Answer · 2016-05-19T10:39:41.727

6

Object.e must be defined on the object's prototype. Like this:

var test = function() {}
test.prototype = { e: function() { return 'e'; } }
var obj = new test();
Object.keys(obj) // returns []
obj.e() // returns 'e'

A way of getting the prototypes keys is simply getting the prototype and the use the Object.keys() function:

Object.keys(Object.getPrototypeOf(obj))

This will however not give you keys of the prototypes prototype.

edited May 19 '16 at 10:39

answered May 19 '16 at 10:22

Emil Ingerslev

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Is there a way to list all property of the object's prototype. – Aman Gupta May 19 '16 at 10:36
Use "for in" as this will list all properties of the object and those inherited by its prototype. – evolutionxbox May 19 '16 at 10:39
Updated my answer. You could also use the `for ... in` as @evolutionxbox says, but if it's a list you want, then you have to build it yourself with the `for ... in` method. – Emil Ingerslev May 19 '16 at 10:41
Look at http://stackoverflow.com/questions/3068534/getting-javascript-object-key-list for good solution on using `for ... in` – Emil Ingerslev May 19 '16 at 10:42

score 3 · Answer 2 · answered May 19 '16 at 10:40

The propety of an JS Object could be

own -- means it is defined on the object directly
inherited -- the property is callable, but is defined in prototype (the Object predecessor)
enumerable / not enumerable -- some properties (like length) are not enumerable, you can define your own non enumerable property with Object.defineProperty()

then iterating functions works on some subset of this properties:

Object.keys() -- returns an array of a given object's own enumerable properties.
Object.getOwnPropertyNames -- returns an array of all properties (enumerable or not) found directly upon a given object

so none of this function iterates over inherited properties. If you wanna iterate all properties without resolution of ownership, then you need for... in cycle, like:

var keys = [];
for (var key in object)
    keys.push(key);

but this will not iterate over non-enumerable properties.

For complete overview of ownership and enumerability see this documentation.

Function not listed under Object.keys or Object.getOwnPropertyNames but can be invoked

2 Answers2