What's the meaning of "=>" in TypeScript? (Fat Arrow)

Question

I just start to learn TypeScript, and I saw there is a lot of code using this sytax =>. I did some research by reading the Specification of TypeScript Version 1.6 and some googling. I still cannot understand the meaning of =>.
To me, it feels like a pointer in C++. But I can't confirm it. If anyone can explain the following examples, that will be great. Thank you!

Below are some examples that I found when I was reading the specification of Typescript :

Object Types

var MakePoint: () => {       x: number; y: number;   }; 

Question: What is this code doing? Creating an object called MakePoint, where x and y fields are number type? Is this a constructor or a function for MakePoint?

Function Types

function vote(candidate: string, callback: (result: string) => any) {    // ...   } 

Question: What is the meaning of => any? Do you have to return a string type?

Can anyone explain to me the difference or the purpose of these examples in plain english? Thank you for your time!

Answer 1

Perhaps you are confusing type information with a function declaration. If you compile the following:

var MakePoint: () => {x: number; y: number;}; 

you will see that it produces:

var MakePoint; 

In TypeScript, everything that comes after the : but before an = (assignment) is the type information. So your example is saying that the type of MakePoint is a function that takes 0 arguments and returns an object with two properties, x and y, both numbers. It is not assigning a function to that variable. In contrast, compiling:

var MakePoint = () => 1; 

produces:

var MakePoint = function () { return 1; }; 

Note that in this case, the => fat arrow comes after the assignment operator.

Answer 2

In a type position, => defines a function type where the arguments are to the left of the => and the return type is on the right. So callback: (result: string) => any means "callback is a parameter whose type is a function. That function takes one parameter called result of type string, and the return value of the function is of type any".

For the expression-level construct, see What's the meaning of "=>" (an arrow formed from equals & greater than) in JavaScript?