Day 9: Function Parameters & Return Values in JavaScript

Parameters are variables listed inside the parentheses of a function definition. They act as placeholders for values you pass into the function.

Think of parameters like the delivery address for a package — the function needs to know where to send or use the data.

Example:

javascriptCopyEditfunction greet(name) {
  console.log(`Hello, ${name}!`);
}

greet("God'sgift");  // Output: Hello, God'sgift!
greet("Samuel");     // Output: Hello, Samuel!

Here, name is a parameter and "God'sgift" or "Samuel" are arguments (the actual values you pass).

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Multiple Parameters

A function can have more than one parameter, separated by commas.

Example:

javascriptCopyEditfunction add(a, b) {
  console.log(a + b);
}

add(3, 4); // Output: 7
add(10, 5); // Output: 15

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Return Values

So far, we’ve used console.log() to display results, but sometimes you want a function to give back a value you can use later.
This is where the return statement comes in.

Example:

javascriptCopyEditfunction multiply(a, b) {
  return a * b;
}

let result = multiply(5, 6);
console.log(result); // Output: 30

The return statement sends the value back to the part of the code that called the function.

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Important Notes on return

• return ends the function immediately — no code after it will run.

• If no return is given, the function returns undefined by default.

Example:

javascriptCopyEditfunction test() {
  return "Done!";
  console.log("This will never run");
}

console.log(test()); // Output: Done!

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Why Use Parameters & Returns?

They make functions reusable, dynamic, and powerful.
Instead of hardcoding values, you can pass any data you want into the same function.

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Summary

• Parameters are placeholders for function input.

• Arguments are the actual values passed.

• return sends a value back from the function.

• Functions without return give undefined.

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📝 Practice Questions

Easy

Write a function square that takes a number as a parameter and returns its square.

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Intermediate

Create a function calculateArea that takes length and width as parameters and returns the area of a rectangle.

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Hard

Write a function isEven that takes a number and returns true if it’s even, and false if it’s odd. Then use this function to log all even numbers from 1 to 20.

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💡 Tomorrow (Day 10), we’ll explore Default Parameters & Arrow Functions to make your code even more flexible.

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