Functions and Type Safety

Functions are the building blocks of any TypeScript application. TypeScript adds powerful type annotations to function parameters, return values, and overloads, making your code more predictable and self-documenting. This lesson covers every aspect of typing functions in TypeScript.

---

Parameter and Return Types

Always annotate function parameters. Return types can often be inferred, but adding them explicitly improves readability:

function add(a: number, b: number): number {
  return a + b;
}

const multiply = (x: number, y: number): number => x * y;

If you omit the return type, TypeScript infers it from the return statements. However, explicit return types catch mistakes early — for example, accidentally returning undefined from a function that should always return a value.

---

Optional Parameters

Use ? to mark a parameter as optional. Optional parameters must come after required ones:

function greet(name: string, title?: string): string {
  if (title) {
    return `Hello, ${title} ${name}`;
  }
  return `Hello, ${name}`;
}

greet('Alice');            // 'Hello, Alice'
greet('Alice', 'Dr');      // 'Hello, Dr Alice'

Inside the function body, an optional parameter has a union type of its declared type and undefined.

---

Default Values