Mastering Function with Default Parameters in TypeScript

Updated: January 7, 2024 By: Guest Contributor Post a comment


In the rich lexicon of TypeScript, default function parameters stand as a beacon of flexibility, allowing developers to set the stage for values unspoken. This tutorial seeks to unravel the art of their utilization.

Introduction to Default Parameters

Function parameters in TypeScript can be initialized with default values. When a function is invoked without passing arguments for such parameters, the default values are utilized, hence ensuring the function operates smoothly without the need for all its parameters explicitly passed every time.

function greet(name: string = 'stranger'): void {
    console.log('Hello, ' + name);

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

Functions with Multiple Default Parameters

More complex scenarios may include multiple parameters, each with their own defaults. Mastery is found in the balance of defining what is essential and what can be safely assumed.

function createUserData(name: string = 'Anonymous', age: number = 0, isActive: boolean = true) {
    return { name, age, isActive };


Combining Default, Optional, and Rest Parameters

Default parameters intermingle with optional and rest parameters, a veritable dance of functionality inviting a myriad of use cases, as TypeScript understands both the spoken and unspoken needs of your functions.

function buildName(firstName: string = 'Will', lastName?: string, ...titles: string[]) {
    let name = titles.join(' ') + ' ' + firstName;
    if (lastName) name += ' ' + lastName;
    return name;

console.log(buildName(undefined, 'Turner', 'Captain'));

Edge Cases Handling

In the tapestry of default parameters, one must also be mindfully aware of potential edge cases and handle them with elegance and precision. Tricky aspects like nullish values or TypeScript’s type system nuances challenge the astute developer to sharpen their understanding.

// Edge case with `null` as it is considered a valid value
defaultName(null); // 'Hello, null'

// Explicitly checking for undefined is usually a safer way to handle defaults
defaultName(name: string | null | undefined) {
    name = (name === undefined || name === null) ? 'stranger' : name;

Default Parameters in Arrow Functions and Methods

Arrow functions and even object or class methods accept default parameters in TypeScript with the same grace and efficiency, demonstrating that elegance is not sacrificed in brevity or in the context of object-oriented constructs.

const User = {
    sayHi: (user: string = 'Visitor') => {
        console.log('Greetings, ' + user);

User.sayHi();  // 'Greetings, Visitor'

Advanced Usage Patterns

Advanced patterns, such as utilizing defaults with destructuring assignments, type constraints, or function overloads, requires a delicate touch to balance power and simplicity, an embodiment of TypeScript’s capability to cater to both straightforward and intricate requirements.

function configureSettings(
    { verbose = true, depth = 1 }:
    { verbose?: boolean, depth?: number } = {}
): void {
    // Configuration logic here


Through the depths of TypeScript’s clever constructs, we have glimpsed the formidable utility of default function parameters. These gentle guides within the language’s ecosystem reward the prudent programmer with powerful, concise, and clear code.