Object methods vs functions and encapsulation in JavaScript

Question

In class-based object oriented languages one of the benefits of objects is encapsulation: every method of an object has access to the object's data.

In JavaScript this benefit of encapsulation doesn't seem to be the same because of the feature of this.

In the example below method1 has access to this without additional binding, but method2 does not.

Is there any reason to use an object method instead of a plain function in the example below if we need to bind both method2 and function2 in order to have access to this.args inside them?

// service declaration

function Service(args) {
    this.args = args;
}

Service.prototype.method1 = function(query) {
    ............
    let res1 = service2.get(query).map(this.method2.bind(this)); // option 1
    let res2 = service2.get(query).map(function2.bind(this));    // option 2
    ............
};

Service.prototype.method2 = function(data) {
    // use args from 'this'
}

function function2(data) {
    // use args from 'this'
}

// service use

let service = new Service(args);
service.method1(req.query).

what is document(data)? What is service2? Do you have a less contrived example? I don't understand what you're trying to do. — Evan Davis
– Evan Davis, Commented Jan 18, 2015 at 18:45
The obvious advantage would be to invoke service.method2() instead of function2.call(service) when it's not necessary to be bound (where there is no big difference) — Bergi
– Bergi, Commented Jan 18, 2015 at 19:07
@kyrylkov: The general approach is to make as few things private as necessary :-) For an OOP design with methods (accessed as properties) the prototype is standard, but for (possibly private) static functions a functional approach, that uses an additional argument instead of this, gains popularity. — Bergi
– Bergi, Commented Jan 18, 2015 at 21:38

rioc0719 · Accepted Answer · 2015-01-18 19:02:04Z

2

I think you should always use methods instead of using bind(this) because when you use external functions, you need to carry those along with that class. Why not just include them as a method on the class?

Also, I don't get the point of the this.method2.bind(this) call. this.method2 is already bound to this because it is a property of this.

Assuming you know some ES6 from your let statement, you could write your code like this:

class Service {
  constructor(args) {
    this.args = args;
  }
  method1(query) {
    let res1 = Service2.get(query).map(this.method2); //btw, what is Service2 here? Is it just an object with methods or does it need to be initialized?
  }
  method2(data) {
    //use data
  }
}
let service = new Service(args);
service.method1(req.query);

edited Jan 18, 2015 at 19:02

answered Jan 18, 2015 at 18:56

rioc0719

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4 Comments

krl Over a year ago

If I use let res1 = Service2.get(query).map(this.method2) without bind on method2 defined via prototype, this ends up being undefined in method2 Is ES6 class behavior different? (I'll check now)

krl Over a year ago

I am on io.js and V8 and ES6 classes are not implemented yet. So the bottom line is that with method2 define via prototype I need bind to access proper this

rioc0719 Over a year ago

@kyrylkov You can use 6to5 to transpile classes to pure ES5. And all methods of a class in ES6 have this bound correctly- I just checked with this dummy code: ` class Service { constructor(args) { this.args = args; } method1(query) { let res1 = [1, 2, 3].map(this.method2); //btw, what is Service2 here? Is it just an object with methods or does it need to be initialized? return res1; } method2(data) { return data * 2; } } let service = new Service({ arg: 1 }); service.method1("data");`

krl Over a year ago

that's a big plus of ES6 classes in this case

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