How to count the number of objects in a constructor javascript

Question

Im working with javascript and would like to know the number of objects inside of a constructor.

I have this:

var EventData = function(ID, Name, StartDate, StartTime, EndDate, EndTime, Location, Notes){
  this.type = "event";
  this.id = ID;
  this.name = Name;
  this.startDate = StartDate;
  this.startTime = StartTime;
  this.endDate = EndDate;
  this.endTime = EndTime;
  this.location = Location;
  this.notes = Notes;
};
EventData.prototype.count = function(){
  return //some code;
};

And i want to call something like this:

var Start = function(){
  var thisEventData = new EventData(1,"Bill", 1,1,1,1, "Home", "N/A");
  console.log(thisEventData.count());
};

Where thisEventData.count() would return 10.

"Number of objects inside of a constructor" doesn't make a lot of sense. You can count the enumerable properties of an object (regardless of where the properties came from) with Object.keys(obj).length. — Pointy
– Pointy, Commented Apr 15, 2015 at 21:09
You don't have to prototype a count function. Is simple as EventData.length — manuerumx
– manuerumx, Commented Apr 15, 2015 at 21:11
I would need 10 if I was to include prototypes with the count. 8 would give me the number of parameters asked. — Devcon
– Devcon, Commented Apr 15, 2015 at 21:28

dfsq · Accepted Answer · 2015-04-15 21:16:16Z

6

The most natural way in this case would be to store parameters count in some property (maybe private). I'm not sure why you need this, but, yea, something like this:

var EventData = function (ID, Name, StartDate, StartTime, EndDate, EndTime, Location, Notes) {
    this._paramsCount = arguments.length;
    this.type = "event";
    this.id = ID;
    this.name = Name;
    this.startDate = StartDate;
    this.startTime = StartTime;
    this.endDate = EndDate;
    this.endTime = EndTime;
    this.location = Location;
    this.notes = Notes;
};
EventData.prototype.count = function () {
    return this._paramsCount;
};

UPD. Based on the edited question, look like you want to calculate number of own properties of the instance. In this case you would use convenient Object.keys method which returns an array of property names.

EventData.prototype.count = function () {
    return Object.keys(this).length;
};

edited Apr 15, 2015 at 21:16

answered Apr 15, 2015 at 21:09

dfsq

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4 Comments

taxicala Over a year ago

I was just about to save my answer exactly as yours, so i've deleted it and upvoted since its a pretty good approach :)

A.B Over a year ago

@dfsq i think he Op means the properties present, not the arguments passed. arguments.length will give 3 if we pass only 3 arguments

dfsq Over a year ago

@A.B Hm.. originally it was parameters, I guess. Looks like you are right.

A.B Over a year ago

still needs confirmation @dfsq may b this is the scenario, +1 fro edit

adeneo · Accepted Answer · 2015-04-15 21:10:03Z

4

Just get the properties of the EventData object as an array with Object.keys, and then the length of the array?

EventData.prototype.count = function(){
  return Object.keys(this).length;
};

answered Apr 15, 2015 at 21:10

adeneo

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Comments

Lucas Trzesniewski · Accepted Answer · 2015-04-15 21:16:18Z

If you want to know the number of canonical parameters, use the Function.length property, in your case:

EventData.length

or

thisEventData.constructor.length

Note that it won't count additional parameters (variable arguments) since it infers the count from the constructor object.

If you want to know the property count of the constructed object, use Object.keys:

Object.keys(thisEventData).length

This will give you the count of the own properties (excluding the prototype chain).

Or just write your own customizable logic if you need something else, for instance if you want non-falsey objects:

function getPropertyCount(obj, withPrototypeChain) {
    var prop;
    var count = 0;

    for (prop in obj) {
        if ((withPrototypeChain || obj.hasOwnProperty(prop)) && obj[prop])
            ++count;
    }

    return count;
}

Jar · Accepted Answer · 2020-04-24 14:08:08Z

0

To build a bit on the accepted answer:

export class Person {
      constructor (name, age) {
        if (arguments.length !== 2) {
          throw new Error('invalid Person argument count')
        }
        this.name = name
        this.age = age 
      }
    }

answered Apr 24, 2020 at 14:08

Jar

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