Type Guards & Narrowing in TypeScript
From the TypeScript cheat sheet · Advanced Types · verified Jul 2026
Type Guards & Narrowing
All the ways to narrow types in conditional blocks
typescript
// typeof
if (typeof x === "string") { x.toUpperCase(); }
// instanceof
if (err instanceof Error) { err.message; }
// in operator
if ("email" in user) { user.email; }
// Custom type predicate
function isString(x: unknown): x is string {
return typeof x === "string";
}💡 Discriminated unions with a "kind" field + switch is the most type-safe branching pattern
⚡ Custom type predicates (is keyword) let you create reusable narrowing functions
📌 The assertNever pattern catches missing switch cases at compile time — essential for unions
🟢 TypeScript narrows automatically with typeof, instanceof, in, truthiness, and equality checks
Continue with TypeScript
Save the full cheat sheet or work through every related task.