Parameters JS.md

Parameters and Return Values

Overview

In JavaScript, parameters and return values define how functions receive input and produce output. Understanding them is crucial for writing reusable and predictable code.

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1. Parameters

Definition

Parameters are variables listed in a function’s definition. They act as placeholders for the values (arguments) that will be passed when the function is called.

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

Here:

• a and b are parameters.
• They represent values supplied during function execution.

Types of Parameters

a. Required Parameters

Must be provided when calling the function.

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

greet("Brandon"); // Output: Hello, Brandon

b. Default Parameters

Used when no argument is provided.

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

greet(); // Output: Hello, Guest

c. Rest Parameters

Capture multiple arguments into an array.

function sum(...numbers) {
  return numbers.reduce((acc, val) => acc + val, 0);
}

console.log(sum(1, 2, 3, 4)); // Output: 10

d. Destructured Parameters

Used for objects or arrays.

function displayUser({ name, age }) {
  console.log(`${name} is ${age} years old`);
}

displayUser({ name: "Alice", age: 25 }); // Output: Alice is 25 years old

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2. Arguments vs Parameters

• Parameters: variables in the function definition.
• Arguments: actual values passed to those parameters when the function is called.

function multiply(x, y) { // x, y → parameters
  return x * y;
}

multiply(4, 5); // 4, 5 → arguments

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3. Return Values

Definition

The return value is the output produced when a function finishes executing. Defined using the return keyword.

function square(num) {
  return num * num;
}

let result = square(4); // result = 16

Characteristics

• The return statement ends the function execution immediately.
• A function without return returns undefined.

function noReturn() {
  let a = 10;
}
console.log(noReturn()); // undefined

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4. Multiple Return Values

A function can return multiple values via arrays or objects.

Using Arrays

function getCoordinates() {
  return [10, 20];
}

const [x, y] = getCoordinates();
console.log(x, y); // 10 20

Using Objects

function getUser() {
  return { name: "Brandon", age: 30 };
}

const { name, age } = getUser();
console.log(name, age); // Brandon 30

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5. Return Values and Control Flow

The return statement stops further execution.

function check(num) {
  if (num > 0) return "Positive";
  if (num < 0) return "Negative";
  return "Zero";
}

console.log(check(5)); // Positive

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6. Best Practices

• Keep parameter lists short (prefer objects for many parameters).
• Use descriptive parameter names.
• Always return consistent types.
• Use default parameters instead of checking for undefined.
• Document both parameters and return values in code comments.

/**
 * Calculates total price with tax.
 * @param {number} price - Base price.
 * @param {number} taxRate - Tax percentage.
 * @returns {number} Total price including tax.
 */
function calculateTotal(price, taxRate = 0.1) {
  return price + price * taxRate;
}

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7. Common Pitfalls

Issue	Description	Example
Missing return	Function doesn’t output anything	function f(x) { x + 1 } → undefined
Wrong parameter order	Arguments mapped incorrectly	function divide(a, b) { return a/b } → divide(2, 10) ≠ divide(10, 2)
Mutating parameters	Changing input objects inside a function	Avoid side effects

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8. Advanced Example

function processOrder({ items, taxRate = 0.08 }) {
  const subtotal = items.reduce((sum, item) => sum + item.price, 0);
  const total = subtotal + subtotal * taxRate;
  return { subtotal, total };
}

const order = {
  items: [
    { name: "Headphones", price: 50 },
    { name: "Keyboard", price: 80 }
  ],
  taxRate: 0.1
};

const { subtotal, total } = processOrder(order);
console.log(subtotal, total); // 130 143

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9. Test Questions

1. Define parameters and arguments and explain the difference.
2. What happens when a function does not include a return statement?
3. Write a function with default and rest parameters to calculate average scores.
4. How can multiple values be returned from one function? Show both array and object methods.
5. Explain why mutating parameters inside functions is discouraged.
6. Implement a function calculateBMI(weight, height) that returns both BMI value and category.
7. How does destructuring in parameters simplify function usage? Provide an example.
8. What is the purpose of default parameters in function definitions?
9. Demonstrate control flow using return in conditional branches.
10. Comment your code using JSDoc to describe parameters and return values for a simple function.

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