Type Guards & Narrowing

Type guards help the compiler figure out which shape a value has at a given point. When a check succeeds, TypeScript narrows the type so you can access the right properties safely:

function process(value: string | number) {
  if (typeof value === 'string') {
    return value.toUpperCase();  // ✅ TypeScript knows it's a string
  }
  return value.toFixed(2);  // ✅ TypeScript knows it's a number
}

Built-in type guards:

typeof – for primitives ('string', 'number', 'boolean')
instanceof – for classes (value instanceof Date)
in – for property checks ('name' in obj)
Array.isArray() – for arrays

Custom type guards:

function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null && 'id' in value;
}

In this category, you'll:

Narrow unions with built-in type guards
Write custom type predicates with value is Type
Use exhaustive switch statements with discriminated unions
Leverage the never type to catch unhandled cases

By the end, you'll be able to narrow any union type safely and teach TypeScript about your own runtime checks.

Number or Text Size

Password or Token

When Is It a Date?

Single or Many

Build a Custom Guard

Exhaustive Result Renderer