Type Guards and Narrowing

Type guards tell TypeScript: what type you have right now.

  ## typeof guard
  
  ```ts
  function print(val: number | string) {
    if (typeof val === "string") {
      console.log(val.toUpperCase());
    } else {
      console.log(val.toFixed(2));
    }
  }
  ```
  
  ## instanceof guard
  
  ```ts
  class User {
    name = "Tom";
  }
  
  function check(x: User | Date) {
    if (x instanceof User) {
      console.log(x.name);
    } else {
      console.log(x.getFullYear());
    }
  }
  ```
  
  ## Custom type guard
  
  ```ts
  type Cat = { meow: () => void };
  type Dog = { bark: () => void };
  
  function isCat(pet: Cat | Dog): pet is Cat {
    return (pet as Cat).meow !== undefined;
  }
  ```
  
  ## Graph
  
  ```mermaid
  flowchart TD
    A[Union] --> B{Check}
    B -->|string| C[String branch]
    B -->|number| D[Number branch]
  ```
  
  ## Remember
  - Guards make unions usable
  - Prevent runtime mistakes