TypeScript Type Narrowing and Type Guards

function process(value: string | number | null) {
    // typeof narrows primitives
    if (typeof value === "string") {
        return value.toUpperCase();  // string
    }
    if (typeof value === "number") {
        return value.toFixed(2);     // number
    }
    // value is null here
    return "no value";
}

// Truthiness narrows null/undefined/falsy
function display(name: string | null) {
    if (name) {
        console.log(name.toUpperCase());  // string — null is excluded
    }
}

class NetworkError extends Error {
    status: number;
    constructor(status: number) {
        super();
        this.status = status;
    }
}

function handleError(error: Error | NetworkError) {
    if (error instanceof NetworkError) {
        console.log(error.status);   // NetworkError properties available
    }
}

// in operator
type Cat = { meow(): void; purr(): void };
type Dog = { bark(): void; fetch(): void };

function interact(animal: Cat | Dog) {
    if ("meow" in animal) {
        animal.meow();   // animal is Cat
    } else {
        animal.bark();   // animal is Dog
    }
}

// Type predicate: "value is User" tells TypeScript to narrow
function isUser(value: unknown): value is User {
    return (
        typeof value === "object" &&
        value !== null &&
        "id" in value &&
        "name" in value
    );
}

function processInput(data: unknown) {
    if (isUser(data)) {
        console.log(data.name);  // TypeScript knows data is User
    }
}

// Useful for validating API responses
async function getUser(id: string) {
    const data = await fetch(`/api/users/${id}`).then(r => r.json());
    if (!isUser(data)) throw new Error("Invalid user data");
    return data;  // type: User
}

type Shape = "circle" | "square" | "triangle";

function getArea(shape: Shape): number {
    switch (shape) {
        case "circle":   return Math.PI * 5 ** 2;
        case "square":   return 5 ** 2;
        case "triangle": return (5 * 4) / 2;
        default:
            const exhaustiveCheck: never = shape;
            // If you add a new Shape without handling it here,
            // TypeScript will show an error at compile time
            throw new Error(`Unhandled shape: ${exhaustiveCheck}`);
    }
}

Exam tipCustom type guards (value is Type) are commonly asked in senior TypeScript interviews. Know that without them, TypeScript can't narrow types based on your own validation logic — the predicate syntax is what tells the compiler about the narrowing.