Methods in ES6 objects: using arrow functions

109
votes

In ES6, both of these are legal:

var chopper = {
    owner: 'Zed',
    getOwner: function() { return this.owner; }
};

and, as shorthand:

var chopper = {
    owner: 'Zed',
    getOwner() { return this.owner; }
}

Is it possible to use the new arrow functions as well? In trying something like

var chopper = {
    owner: 'John',
    getOwner: () => { return this.owner; }
};

or

var chopper = {
    owner: 'John',
    getOwner: () => (this.owner)
};

I get a error messages suggesting that the method does not have access to this. Is this just a syntax issue, or can you not use fat-pipe methods inside of ES6 objects?

6
javascriptecmascript-6ecmascript-harmony
One of the biggest points of the new function syntax was that it treats this differently. It's defined by the lexical environment in which the function was created, which means the this value where you create the chopper variable will be the this value of the function. In other words, it won't be referencing the chopper object. – user1106925
While using the fat arrow syntax? Only if you alter the this value by first creating the chopper object, then doing the assignment in a function that has this pointing to that object. This can be accomplished pretty cleanly with a constructor function. – user1106925
This demo will run in Firefox. Chrome doesn't have it yet. jsfiddle.net/bfyarxfe – user1106925
@fox, you must use 'use strict' on that jsfiddle. – Walter Chapilliquen - wZVanG
@fox: It works fine in a supported environment. Firefox doesn't yet have complete support. Try it in Continuum and console.log() the result of the method call. It works. – user1106925

6 Answers

182
votes

Arrow functions are not designed to be used in every situation merely as a shorter version of old-fashioned functions. They are not intended to replace function syntax using the function keyword. The most common use case for arrow functions is as short "lambdas" which do not redefine this, often used when passing a function as a callback to some function.

Arrow functions cannot be used to write object methods because, as you have found, since arrow functions close over the this of the lexically enclosing context, the this within the arrow is the one that was current where you defined the object. Which is to say:

// Whatever `this` is here...
var chopper = {
    owner: 'Zed',
    getOwner: () => {
        return this.owner;    // ...is what `this` is here.
    }
};

In your case, wanting to write a method on an object, you should simply use traditional function syntax, or the method syntax introduced in ES6:

var chopper = {
    owner: 'Zed',
    getOwner: function() {
        return this.owner;
    }
};

// or

var chopper = {
    owner: 'Zed',
    getOwner() {
        return this.owner;
    }
};

(There are small differences between them, but they're only important if you use super in getOwner, which you aren't, or if you copy getOwner to another object.)

There was some debate on the es6 mailing list about a twist on arrow functions which have similar syntax but with their own this. However, this proposal was poorly received because that is mere syntax sugar, allowing people to save typing a few characters, and provides no new functionality over existing function syntax. See the topic unbound arrow functions.

14
votes

In this line getOwner: () => this.owner should be:

var chopper = {
    owner: 'John',
    getOwner: () => this.owner
}; //here `this` refers to `window` object.

You would have to declare this into a function:

var chopper = {
    owner: 'John',
    getOwner() { return this.owner }
};

Or:

var chopperFn = function(){

    this.setOwner = (name) => this.owner = name;
    Object.assign(this,{
        owner: 'Jhon',
        getOwner: () => this.owner,
    })

}

var chopper = new chopperFn();
console.log(chopper.getOwner());
chopper.setOwner('Spiderman');
console.log(chopper.getOwner());
2
votes

If you have to use arrow function, you can change this to chopper,

var chopper = {
  owner: "John",
  getOwner: () => chopper.owner
};

Although this is not best practice, when you change the object name, you have to change this arrow function.

1
votes

Another tip, in strict mode, this still refers to Window instead of undefined.

  (() => {
    "use strict";
    console.log(this); // window
  })();

  (function () {
    "use strict";
    console.log(this); // undefined
  })();
0
votes

This inside arrow function doesn't reflect context of the object. Instead it gives the context where the object method is called.

Check this, This gives some insight about when to use arrow and when not. https://dmitripavlutin.com/when-not-to-use-arrow-functions-in-javascript/

0
votes

A quick tip that I follow to use arrow functions.

  • Use non-arrow functions for methods that will be using object.method() syntax. (Those are functions that will receive meaningful this value from their caller.)
  • Use arrow function for almost everything else.