Union Types

Union Types

Building on Optional Properties: In the previous lesson, you saw that an optional property like age?: number actually has the type number | undefined — that | is a union! You have already been using union types without realizing it. Now let's explore them fully.

A union type means a value can be one of several types, written with |:

let input: string | number;
input = "hello";  // OK
input = 42;       // OK
input = true;     // Error: 'boolean' not in the union

Unions in Functions

function stringify(val: string | number | boolean): string {
    return String(val);
}

console.log(stringify("hello"));  // hello
console.log(stringify(42));       // 42
console.log(stringify(true));     // true

Unions with null and undefined

A common union is T | null for values that might be absent:

function find(id: number): string | null {
    if (id === 1) return "Alice";
    return null;
}

const name = find(1);   // string | null
const missing = find(2); // string | null

Your Task

Write function describe(val: string | number): string that returns:

• "string of length N" if val is a string (where N is the length)
• "number: N" if val is a number