TypeScript Optional and Default Parameters Explained

Welcome back! I am Mihir, and in this lesson we will learn optional and default parameters in TypeScript.

These let you write functions where some arguments are not always required.

Optional Parameters

Use ? after the parameter name.

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

  return "Hello, guest";
}

Both calls are valid:

greet("Mihir");
greet();

name?: string means name can be string or undefined.

Optional Parameters Are Possibly Undefined

This is unsafe:

function greet(name?: string): string {
  return name.toUpperCase();
}

TypeScript warns because name might be undefined.

Correct:

function greet(name?: string): string {
  return name?.toUpperCase() ?? "GUEST";
}

Required Parameters Come First

Optional parameters should come after required parameters.

function createUser(name: string, role?: string): string {
  return `${name}: ${role ?? "user"}`;
}

This is easy to call:

createUser("Mihir");
createUser("Mihir", "admin");

Avoid putting optional parameters before required ones because function calls become confusing.

Default Parameters

Default parameters provide a fallback value.

function greet(name: string = "guest"): string {
  return `Hello, ${name}`;
}

These both work:

greet("Mihir");
greet();

When no argument is passed, name becomes "guest".

Default Parameters and Type Inference

TypeScript can infer the parameter type from the default value.

function setPageSize(size = 10): number {
  return size;
}

TypeScript infers size as number.

You can still write the type explicitly:

function setPageSize(size: number = 10): number {
  return size;
}

Passing undefined Uses the Default

Default values are used when the argument is undefined.

function formatName(name: string = "guest"): string {
  return name.toUpperCase();
}

formatName(undefined);

This returns:

GUEST

But null is different:

formatName(null);

With strict settings, TypeScript warns because null is not a string.

Optional vs Default Parameters

Optional parameter:

function greet(name?: string): string {
  return name ?? "guest";
}

Default parameter:

function greet(name: string = "guest"): string {
  return name;
}

Use a default parameter when you know the fallback value immediately.

Use an optional parameter when the function needs custom missing-value logic.

Optional Object Parameters

Optional parameters can be objects too.

type SearchOptions = {
  limit?: number;
  sortBy?: "newest" | "oldest";
};

function search(query: string, options?: SearchOptions): void {
  console.log(query);
  console.log(options?.limit ?? 10);
  console.log(options?.sortBy ?? "newest");
}

This gives callers flexibility while keeping the structure typed.

Default Object Parameters

type SearchOptions = {
  limit: number;
  sortBy: "newest" | "oldest";
};

function search(
  query: string,
  options: SearchOptions = { limit: 10, sortBy: "newest" }
): void {
  console.log(query, options.limit, options.sortBy);
}

This works when you want a full default object.

Common Mistake: Forgetting undefined

function printDiscount(code?: string): void {
  console.log(code.toUpperCase());
}

code might be undefined.

Correct:

function printDiscount(code?: string): void {
  if (code) {
    console.log(code.toUpperCase());
  }
}

Quick Reference Summary

Practice

Create a function with optional and default values:

function createCourse(title: string, lessons: number = 10, category?: string): string {
  return `${title} has ${lessons} lessons in ${category ?? "general"}.`;
}

console.log(createCourse("TypeScript"));
console.log(createCourse("TypeScript", 20, "programming"));

Try using category.toUpperCase() without checking it.

What You've Learned

You now understand:

• How optional parameters work
• Why optional parameters may be undefined
• How default parameters work
• How TypeScript infers default parameter types
• The difference between optional and default parameters
• How to type optional object parameters
• How to avoid unsafe access

What's Next?

In the next lesson, we will learn Rest Parameters in TypeScript.

We will type functions that accept any number of arguments.

Need Help?

• Have questions, confusion, or want to know more? Contact me

Optional and default parameters make functions nicer to call, as long as missing values are handled clearly.