Union Types & Narrowing

Union types

A union type says a value can be one of several types, separated by |:

let id: string | number;
id = "abc-123"; // ✓
id = 42;        // ✓
id = true;      // ❌ Error

This is useful when a function accepts or returns values of different shapes.

Type narrowing

When you have a union, TypeScript needs help knowing which specific type you're working with. Use typeof or in checks:

function formatId(id: string | number): string {
  if (typeof id === "string") {
    return id.toUpperCase(); // TypeScript knows id is string here
  }
  return id.toFixed(0);     // TypeScript knows id is number here
}

Literal types

You can also union specific string/number values to build an enum-like type:

type Direction = "north" | "south" | "east" | "west";

function move(dir: Direction) {
  console.log("Moving", dir);
}

move("north"); // ✓
move("up");    // ❌ Error

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Your task

1. Export a function formatValue that accepts a string | number and:

   • If it's a string: returns it in UPPERCASE
   • If it's a number: returns it multiplied by 2

2. Export a type alias Status that is a union of the string literals "active", "inactive", and "pending".