Explain TypeScript type narrowing and the different ways to narrow types at runtime

Question

At a product company interview:

> "We have a function that handles different response types from our API. Each response has a different shape based on the `type` field. How do we safely handle this in TypeScript?"

Interview OS Community · Accepted Answer

## Type Narrowing Techniques

### 1. typeof (primitives)
```typescript
function process(value: string | number) {
  if (typeof value === "string") {
    return value.toUpperCase(); // TypeScript knows it's string
  }
  return value.toFixed(2); // TypeScript knows it's number
}
```

### 2. instanceof (classes)
```typescript
function handleError(error: Error | ValidationError | AuthError) {
  if (error instanceof ValidationError) {
    return { status: 400, message: error.field }; // Access .field
  }
  if (error instanceof AuthError) {
    return { status: 401, message: error.action }; // Access .action
  }
  return { status: 500, message: error.message };
}
```

### 3. Discriminated Unions (BEST for API responses)
```typescript
type ApiResponse =
  | { type: "success"; data: User[] }
  | { type: "error"; error: string; code: number }
  | { type: "loading" };

function handleResponse(res: ApiResponse) {
  switch (res.type) {
    case "success":
      return res.data; // TypeScript knows data exists
    case "error":
      return res.code; // TypeScript knows code exists
    case "loading":
      return "Loading...";
  }
}
```

### 4. Type Predicates
```typescript
function isUser(value: unknown): value is User {
  return typeof value === "object" && value !== null && "email" in value;
}

if (isUser(data)) {
  console.log(data.email); // Safe!
}
```

### 5. Assertion Functions
```typescript
function assertDefined<T>(value: T | undefined): asserts value is T {
  if (value === undefined) throw new Error("Expected defined value");
}

const user = users.find(u => u.id === id);
assertDefined(user);
console.log(user.name); // TypeScript knows user is defined
```

| Technique | Best For | Runtime Cost |
|-----------|----------|-------------|
| typeof | Primitives | None |
| instanceof | Class hierarchies | None |
| Discriminated union | API responses, events | None |
| Type predicate | Custom validation | Check cost |
| Assertion | Invariant enforcement | Throws on fail |

Interview OS

Explain TypeScript type narrowing and the different ways to narrow types at runtime

Question Details

Suggested Solution

Type Narrowing Techniques

1. typeof (primitives)

2. instanceof (classes)

3. Discriminated Unions (BEST for API responses)

4. Type Predicates

5. Assertion Functions

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