ES6 arrow functions not working on the prototype?

When ES6 Arrow functions don't seem to work for assigning a function to an object with prototype.object. Consider the following examples:

function Animal(name, type){
 this.name = name;
  this.type = type;
  this.toString = () => `${this.name} is a ${this.type}`;

}
var myDog = new Animal('Max', 'Dog');
console.log(myDog.toString()); //Max is a Dog

Using the arrow function explicitly in the object definition works, but using the arrow functions with the Object.prototype syntax does not:

function Animal2(name, type){
  this.name = name;
  this.type = type;
}
Animal2.prototype.toString = () => `${this.name} is a ${this.type}`;

var myPet2 = new Animal2('Noah', 'cat');
console.log(myPet2.toString()); //is a undefined

Just as a proof of concept, using the Template string syntax with Object.prototype syntax does work:

function Animal3(name, type){
  this.name = name;
  this.type = type;
}
Animal3.prototype.toString = function(){ return `${this.name} is a ${this.type}`;}

var myPet3 = new Animal3('Joey', 'Kangaroo');
console.log(myPet3.toString()); //Joey is a Kangaroo

Am I missing something obvious? I feel that example 2 should work logically, but I am puzzled by the output. I'm guessing it is a scoping issue, but I am thrown off by the output 'is a undefined'.

ES6 Fiddle

Answers:

Answer

Arrow functions provide a lexical this. It uses the this that is available at the time the function is evaluated.

It is logically equivalent to (the following isn't valid code since you can't have a variable named this):

(function(this){
   // code that uses "this"
 })(this)

In your 1st example the arrow function is within the constructor, and this points to the newly generated instance.

In your 3rd example, an arrow function isn't used and standard this behavior works as always (the this in the function scope).

In your 2nd example, you use an arrow function but at the scope it's evaluated, this is global / undefined.

Answer

Regular function returns a reference to the current JavaScript Object but the arrow function returns the reference to the global window object.

Regular functions are working well with objects using the new keyword. They have the constructor function by which values can be initialized during object creation. It can be managed using the prototype chaining but the arrow function does not have constructor function, prototype chaining. They are not working well with objects. They can not be used with the new keyword for assigning memory.

In your first example, you write your arrow key function inside the regular function, then you will get the output.

function Animal2(name, type){
    this.name = name;
    this.type = type;
}
Animal2.prototype.toString = function(){
    return () => `${this.name} is a ${this.type}`;
}

var myPet2 = new Animal2('Noah', 'cat');
console.log(myPet2.toString()()); //Noah is a cat

Reference: Difference between regular function and arrow key function

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