Functions as types

As those familiar with JavaScript will know, functions themselves are values, and therefore, they have a type.

Let's take two functions, multiply and echoString as examples:

function multiply(x: number, y: number): number {
  return x * y;
}

function echoString(input: string): string {
  return input;
}

We then declare our function type definitions using the following syntax:

let myMultiply: (val1: number, val2: number) => number;
let myEchoString: (val: string) => string;

// Sanity check
myEchoString = echoString; // OK
myMultiply = echoString; // Error: type '(input: string) => string' is not assignable to type '(val1: number, val2: number) => number'