Arrow Functions

Lovingly called the fat arrow (because -> is a thin arrow and => is a fat arrow) and also called a lambda function (because of other languages). Another commonly used feature is the fat arrow function ()=>something. The motivation for a fat arrow is:

1. You don't need to keep typing function
2. It lexically captures the meaning of this
3. It lexically captures the meaning of arguments

For a language that claims to be functional, in JavaScript you tend to be typing function quite a lot. The fat arrow makes it simple for you to create a function

var inc = (x)=>x+1;

this has traditionally been a pain point in JavaScript. As a wise man once said "I hate JavaScript as it tends to lose the meaning of this all too easily". Fat arrows fix it by capturing the meaning of this from the surrounding context. Consider this pure JavaScript class:

function Person(age) {
    this.age = age
    this.growOld = function() {
        this.age++;
    }
}
var person = new Person(1);
setTimeout(person.growOld,1000);

setTimeout(function() { console.log(person.age); },2000); // 1, should have been 2

If you run this code in the browser this within the function is going to point to window because window is going to be what executes the growOld function. Fix is to use an arrow function:

function Person(age) {
    this.age = age
    this.growOld = () => {
        this.age++;
    }
}
var person = new Person(1);
setTimeout(person.growOld,1000);

setTimeout(function() { console.log(person.age); },2000); // 2

The reason why this works is the reference to this is captured by the arrow function from outside the function body. This is equivalent to the following JavaScript code (which is what you would write yourself if you didn't have TypeScript):

function Person(age) {
    this.age = age
    var _this = this;  // capture this
    this.growOld = function() {
        _this.age++;   // use the captured this
    }
}
var person = new Person(1);
setTimeout(person.growOld,1000);

setTimeout(function() { console.log(person.age); },2000); // 2

Note that since you are using TypeScript you can be even sweeter in syntax and combine arrows with classes:

class Person {
    constructor(public age:number) {}
    growOld = () => {
        this.age++;
    }
}
var person = new Person(1);
setTimeout(person.growOld,1000);

setTimeout(function() { console.log(person.age); },2000); // 2

Tip: Arrow Function Need

Beyond the terse syntax, you only need to use the fat arrow if you are going to give the function to someone else to call. Effectively:

var growOld = person.growOld;
// Then later someone else calls it:
growOld();

If you are going to call it yourself, i.e.

person.growOld();

then this is going to be the correct calling context (in this example person).

Tip: Arrow Function Danger

In fact if you want this to be the calling context you should not use the arrow function. This is the case with callbacks used by libraries like jquery, underscore, mocha and others. If the documentation mentions functions on this then you should probably just use a function instead of a fat arrow. Similarly if you plan to use arguments don't use an arrow function.

Tip: Arrow functions with libraries that use this

Many libraries do this e.g jQuery iterables (one example http://api.jquery.com/jquery.each/) will use this to pass you the object that it is currently iterating over. In this case if you want to access the library passed this as well as the surrounding context just use a temp variable like _self like you would in the absence of arrow functions.

let _self = this;
something.each(function() {
    console.log(_self); // the lexically scoped value
    console.log(this); // the library passed value
});