TypeScript Function Types

Functions are core of every app. TypeScript makes them safe.

  ## Basic typing
  
  ```ts
  function add(a: number, b: number): number {
    return a + b;
  }
  ```
  
  ## Optional params
  
  ```ts
  function greet(name: string, age?: number) {
    console.log(name, age);
  }
  ```
  
  ## Default params
  
  ```ts
  function greet(name: string = "Guest") {
    console.log(name);
  }
  ```
  
  ## Function type variable
  
  ```ts
  let calc: (a: number, b: number) => number;
  
  calc = (x, y) => x + y;
  ```
  
  ## Remember
  - Always type parameters and returns
  - Makes APIs predictable