ES6 functions, arrow functions and 'this' in an ES6 class

It's important to know that this syntax:

class A {
  method = () => {}
}

is just syntactic sugar for creating an instance method in the class constructor:

class A {
  constructor() {
    this.method = () => {}
  }
}

Note: This syntax is not an official part of the JavaScript language yet (currently in stage 3) so you must use a transpiler like Babel to handle it.

The value of this within method is the class A because that is what this points to in the constructor (since arrow functions inherit the context from the scope they are defined in):

class A {
  constructor() {
    this.method = () => this;
  }
}

const instance = new A();
console.log(instance.method() === instance); // true

Defining a regular (non-arrow function) method on the class creates a method on the class prototype (not instance) but sets no rules on what this will be (since this is dynamic in JS and depends on how a function is called, not how it's defined).

class A {
  method() {}
}

console.log(new A().method === A.prototype.method); // true

If methods defined in either of these ways are called on the class instance (via the .), as per the rule of how this is bound when a function is called as a method of an object, this will point to the class instance in both cases:

class A {
  constructor() {
    this.methodOnInstance = () => this;
  }
  methodOnPrototype() { return this; }
}

const instance = new A();
console.log(
  instance.methodOnInstance() === instance.methodOnPrototype(), // true
  instance.methodOnPrototype() === instance // true
);

One major difference between the two method declarations above is that the instance method has this always fixed to the class instance while the class (prototype) method does not (we can change it by using Function.prototype.apply or Function.prototype.call)

class A {
  constructor() {
    this.methodOnInstance = () => this;
  }
  methodOnPrototype() { return this; }
}

const instance = new A();
console.log(
  instance.methodOnInstance() === instance.methodOnPrototype(), // true
  instance.methodOnPrototype.call('new this') === 'new this' // true
);

A common occurrence where the this changes is within an event handler, where the event handler calls the function passed into it and binds the context to the element on which the event happened (so overrides the value of this to be the element that was clicked or whatever the event was)

This happens in React as well for all (synthetic) DOM event handlers.

Therefore, if we want our method's context to always point to the instance of the React component, we can use the instance method.

Another way of restricting the context but not using the special instance method syntax that requires Babel is to directly create an instance method ourselves by creating a new function from the class (prototype) method with a bound context (using Function.prototype.bind):

class A {
  constructor() {
    this.methodOnInstance = this.methodOnPrototype.bind(this);
  }
  methodOnPrototype() { return this; }
}

const instance = new A();
console.log(
  instance.methodOnInstance() === instance.methodOnPrototype(), // true
  instance.methodOnPrototype() === instance // true
);

This allows us to arrive to the same result as using the special instance method syntax but with the currently available tools (ES2017 and under).

If for some reason we want a method that is always bound to something that is not an instance of the class, we can do that as well:

class A {
  constructor() {
    this.method = this.method.bind(console);
  }
  method() { return this; }
}

const instance = new A();
console.log(
  instance.method() === console // true
);