JavaScript - Default Parameters in JavaScript

1. Default Parameters in JavaScript 

Default parameters allow you to set a default value for a function parameter.
If an argument is not provided or is undefined, JavaScript uses the default value.

Syntax

function functionName(parameter = defaultValue) {
    // Code to execute
}

Example 1: Basic Default Parameter

function greet(name = "Guest") {
    console.log(`Hello, ${name}!`);
}

greet("Alice");   // Argument provided
greet();          // No argument provided

Output:

Hello, Alice!
Hello, Guest!

  • If you don’t pass name, it defaults to "Guest".

Example 2: Multiple Default Parameters

function introduce(name = "Anonymous", age = 18) {
    console.log(`${name} is ${age} years old.`);
}

introduce("Bob", 25);  // Both arguments passed
introduce("Emma");     // Only name passed
introduce();           // No arguments passed

Output:

Bob is 25 years old.
Emma is 18 years old.
Anonymous is 18 years old.

Example 3: Default Parameters with Expressions

You can set dynamic default values:

function calculatePrice(price, tax = price * 0.1) {
    return price + tax;
}

console.log(calculatePrice(100));    // Uses default tax
console.log(calculatePrice(200, 50)); // Uses custom tax

Output:

110
250

Example 4: Default Parameter vs undefined

If you pass undefined, the default value is used.
But if you pass null, the default won’t be used.

function show(message = "Hello") {
    console.log(message);
}

show();             // "Hello"
show(undefined);    // "Hello"
show(null);         // null
show("Hi!");        // "Hi!"

Output:

Hello
Hello
null
Hi!

2. Return Values in JavaScript ✅

A return value is the data a function sends back when it finishes execution.

Syntax

function functionName(parameters) {
    return value;
}

  • return stops function execution immediately.

  • Without return, the function returns undefined.

Example 1: Returning a Value

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

const result = add(5, 10);
console.log(result);

Output:

15

  • The return statement sends the result back to the caller.

Example 2: Function Without Return

function multiply(a, b) {
    console.log(a * b);
}

const result = multiply(5, 4);
console.log(result);

Output:

20
undefined

  • No return → function returns undefined.

Example 3: Using return to Exit Early

function checkAge(age) {
    if (age < 18) {
        return "Access Denied";
    }
    return "Access Granted";
}

console.log(checkAge(16));
console.log(checkAge(20));

Output:

Access Denied
Access Granted

  • Once return runs, the function stops executing.

Example 4: Returning Multiple Values

JavaScript can only return one value, but we can use arrays or objects:

Using Arrays

function getCoordinates() {
    return [40.7128, -74.006];
}

const [lat, lng] = getCoordinates();
console.log(lat, lng);

Output:

40.7128 -74.006

Using Objects

function getUser() {
    return { name: "Alice", age: 25 };
}

const { name, age } = getUser();
console.log(name, age);

Output:

Alice 25

3. Combining Default Parameters + Return Values

function calculateBill(amount, tax = 0.1, tip = 0.05) {
    return amount + amount * tax + amount * tip;
}

console.log(calculateBill(100));         // Uses default tax & tip
console.log(calculateBill(100, 0.2));    // Custom tax, default tip
console.log(calculateBill(100, 0.2, 0)); // Custom tax, no tip

Output:

115
125
120

4. Quick Summary

Feature Default Parameters Return Values
Definition Assigns a default value if no argument is provided Sends data back from a function
Introduced In ES6 Always existed
When Used When arguments might be missing When you need a function to give a result
Returns undefined If argument = undefined, default used If no return provided
Use Case Optional parameters Reusable calculations, passing data