Arrow functions

Arrow functions allow you to shorten function declarations, from function() {} to simply () => {}. Indeed, you can replace a line like this:

SomeEmitter.on('message', function(message) { console.log(message) });

To:

SomeEmitter.on('message', message => console.log(message));

Here, we lose both the brackets and curly braces, and the tighter code works as expected. 

Another important feature of arrow functions is they are not assigned their own this—arrow functions inherit this from the call site. For example, the following code does not work:

function Counter() {
    this.count = 0;

    setInterval(function() {
        console.log(this.count++);
    }, 1000);
}

new Counter();

The function within setInterval is being called in the context of setInterval, rather than the Counter object, so this does not have any reference to count. That is, at the function call site, this is a Timeout object, which you can check yourself by adding console.log(this) to the prior code.

With arrow functions, this is assigned at the point of definition. Fixing the code is easy:

setInterval(() => { // arrow function to the rescue!
  console.log(this);
  console.log(this.count++);
}, 1000);
// Counter { count: 0 }
// 0
// Counter { count: 1 }
// 1
// ...