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