Why is “this” in an ES6 class not implicit?

Question

I know that ES6 solved a lot of problems that existed with the this keyword in ES5, such as arrow functions and classes.

My question relates to the usage of this in the context of an ES6 class and why it has to be written explicitly. I was originally a Java developer and I come from a world where the following lines of code were perfectly natural.

class Person {
  private String myName;

  public Person() { 
    myName = "Heisenberg";
  }

  public void sayMyName() {
    System.out.println("My name is " + myName);
  }
}

The compiler will always refer to the value of the field myName, unless it has a local variable with the name myName defined in the scope of the method.

However, once we convert this code to ES6:

class Person {

  constructor() {
    this.myName = "Heisenberg";
  }

  sayMyName() {
    console.log(`My name is ${myName}`);
  }
}

This won't work and it will throw an Uncaught ReferenceError: myName is not defined. The only way to fix this is to throw in an explicit this reference:

console.log(`My name is ${this.myName}`)

I understand the need for this in the constructor since ES6 classes don't allow your fields to be defined outside of the constructor, but I don't understand why the Javascript engines can't (or won't, because of the standard) do the same as Java compilers can in the case of sayMyName

Answer 1

Maybe I will not directly answer your question, but I'll try to direct the way you should think about JS class keyword.

Under the cover there is no magic about it. Basically it's a syntatic sugar over prototypal inheritance that was since the beginning of JavaScript.

To read more about classes in JS click here and here.

As of why you need explicitly write this is that because in JS it's always context sensitive, so you should refer to exact object. Read more.