JavaScript Default Parameters

Introduction

When working with functions in JavaScript, you often pass values (arguments) to control how a function behaves. But what happens if a value is not provided? Without proper handling, this can lead to unexpected results like undefined or errors.

To solve this problem, JavaScript introduced default parameters in ES6 (ECMAScript 2015). Default parameters allow you to assign a default value to a function parameter if no argument (or undefined) is passed.

This feature makes your code more robust, readable, and easier to maintain.

In this guide, you’ll learn everything about JavaScript default parameters, including syntax, examples, real-life use cases, common mistakes, and interview questions.

What are Default Parameters?

Default parameters allow you to initialize function parameters with default values if no value is provided.

Syntax of Default Parameters

function functionName(param1 = defaultValue, param2 = defaultValue) {
   // code
}

Basic Example:

function greet(name = "Guest") {
   console.log("Hello " + name);
}

greet(); // Hello Guest

Output:

Note: If no argument is passed, “Guest” is used.

Why Use Default Parameters?

There are many reasons to use Default Parameters.

1. Prevent undefined values
2. Improve code readability
3. Reduce the need for conditional checks
4. Make functions more reliable
5. Simplify function logic

When Default Parameters Work

Default values are used when:

1. No argument is passed
2. The argument is undefined

Example:

function test(x = 10) {
   console.log(x);
}

test();          // 10
test(undefined); // 10
test(null);      // null

Output:

Note: null does NOT trigger the default value.

Default Parameters vs Old Method

Before ES6, developers used manual checks.

Old Way:

function greet(name) {
   name = name || "Guest";
   console.log(name);
}
greet();

Output:

Modern Way:

function greet(name = "Guest") {
   console.log(name);
}
greet();

Output:

Multiple Default Parameters

When you pass multiple default parameters in the function.

Example:

function calculate(a = 10, b = 20) {
   return a + b;
}

console.log(calculate()); // 30

Output:

Default Parameters with Functions

You can use functions as default values.

function getValue() {
   return 10;
}

function test(x = getValue()) {
   console.log(x);
}

test();

Output:

Real-Life Examples

You will see some real-life examples

1. Implement Pagination

function paginate(page = 1, limit = 10) {
   console.log(`Page: ${page}, Limit: ${limit}`);
}

2. API Request

function fetchData(url, method = "GET") {
   console.log(`Fetching ${url} with ${method}`);
}

Common Mistakes

Beginners make some common mistakes.

1. Passing null instead of undefined

function test(x = 10) {
   console.log(x);
}

test(null); // null

Output:

2. Incorrect Parameter Order

In JavaScript, parameters with default values should generally come after normal parameters.

❌ Incorrect Parameter Order

function greet(name = "Guest", age) {
  console.log(name, age);
}

✅ Correct Parameter Order

function greet(age, name = "Guest") {
  console.log(name, age);
}

greet(35);             

greet(35, "John");     

Output:

Conclusion

JavaScript default parameters are a powerful feature that makes your functions more flexible and reliable. They eliminate the need for manual checks and help prevent unexpected undefined values.

By using default parameters effectively, you can write cleaner, more maintainable, and professional JavaScript code.

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