📘 JavaScript Default Parameters (2025 Guide)

Learn how to simplify function behavior using default parameters in JavaScript.

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🚀 Introduction – What Are Default Parameters?

Default parameters in JavaScript allow function parameters to have default values if no argument or undefined is passed during function invocation.

✅ Introduced in ES6 (ECMAScript 2015), default parameters make functions more robust and concise by eliminating the need for manual checks.

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📌 Syntax of Default Parameters

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

greet();            // Output: Hello, Guest!
greet("Vaibhav");   // Output: Hello, Vaibhav!

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🔍 How Default Parameters Work

• The default value is assigned only if the parameter is undefined.
• Falsy values like "", 0, or null do not trigger the default assignment.

✅ Example with Multiple Parameters

function multiply(a, b = 2) {
  return a * b;
}

console.log(multiply(5));    // Output: 10
console.log(multiply(5, 3)); // Output: 15

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🧪 Default Parameters with Expressions

You can use expressions, including functions or other variables, as default values.

function getDefault() {
  return 100;
}

function calculate(price = getDefault()) {
  return price * 2;
}

console.log(calculate());     // Output: 200
console.log(calculate(50));   // Output: 100

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⚠️ Important Notes & Best Practices

• Order matters: Always place default parameters after non-default ones.
• Avoid using default parameters for required values.
• Functions can have multiple default parameters, and they may reference earlier parameters.

✅ Example Referencing Previous Parameter

function createUser(name, role = name + "_admin") {
  return { name, role };
}

console.log(createUser("vaibhav")); 
// Output: { name: 'vaibhav', role: 'vaibhav_admin' }

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✅ Use Cases of Default Parameters

Scenario	Benefit
Optional arguments in functions	Cleaner syntax, no if checks
Reusable utility functions	Easier configuration
API request functions	Set default headers, timeouts, etc.

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🧾 Summary

JavaScript’s default parameters simplify code by allowing developers to provide fallback values directly in function definitions. They:

• Eliminate repetitive if/else checks
• Improve function readability and reusability
• Are part of ES6, supported in all modern browsers

🧠 Pro Tip: Always keep required parameters first and use defaults for optional values.

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❓ Frequently Asked Questions (FAQ)

🔹 Q1. What happens if I pass null instead of undefined?

A: Default values are only applied when the argument is undefined. If null is passed, the default will not be used.

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🔹 Q2. Can I use a function as a default parameter?

A: Yes. You can use a function call or even reference earlier parameters in the default expression.

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🔹 Q3. Are default parameters supported in all browsers?

A: Default parameters are supported in modern browsers (Chrome, Firefox, Edge, Safari). Older versions like IE do not support them natively.

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🔹 Q4. What’s the difference between a = 5 inside a function vs as a default parameter?

A: a = 5 inside a function reassigns the variable every time; function fn(a = 5) only assigns 5 if a is undefined.

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🔹 Q5. Can default parameters reference other parameters?

A: Yes, but only parameters defined before them.

function example(a, b = a * 2) {
  return b;
}

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