📘 JavaScript Function Definitions

✅ 1. What Is a Function Definition?

A function definition is how you create a function — specifying its name, parameters, and the block of code it should run when called.

📌 2. Ways to Define Functions in JavaScript

A. Function Declaration (Named Function)

✅ Hoisted (can be used before it’s defined)

jsCopyEditfunction greet() {
  console.log("Hello!");
}

greet(); // "Hello!"

B. Function Expression

A function assigned to a variable.

❌ Not hoisted (must be defined before use)

jsCopyEditconst sayHello = function() {
  console.log("Hi there!");
};

sayHello(); // "Hi there!"

C. Arrow Function (ES6)

✅ Shorter syntax
❌ No this, arguments, or super binding (not suitable as methods)

jsCopyEditconst add = (a, b) => a + b;

console.log(add(2, 3)); // 5

If one parameter, you can omit parentheses:

jsCopyEditconst square = x => x * x;

If no parameters, use () =>:

jsCopyEditconst greet = () => console.log("Hi!");

D. Anonymous Function

A function without a name, often used as a callback.

jsCopyEditsetTimeout(function() {
  console.log("This runs after 1 second");
}, 1000);

E. Immediately Invoked Function Expression (IIFE)

Runs immediately after being defined.

jsCopyEdit(function() {
  console.log("I run automatically!");
})();

F. Constructor Function (Less Common Today)

Used with the new keyword to create objects.

jsCopyEditfunction Person(name) {
  this.name = name;
}

const p = new Person("Alice");
console.log(p.name); // "Alice"

📌 3. Function Parameters and Arguments

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

greet("Tom"); // "Hello, Tom"

Use default parameters:

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

📌 4. Return Values

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

let result = add(3, 4); // 7

🧠 Best Practices

• Use arrow functions for short callbacks.
• Use named functions when you need hoisting or better stack traces.
• Use default parameters to avoid undefined errors.
• Keep functions small and single-purpose.

🧪 Practice Challenges

1. Define a function multiply(x, y) that returns the product.
2. Convert a traditional function to an arrow function.
3. Create an IIFE that logs “Ready to go!”

📘 Summary Table

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• 📄 A PDF or slide version of this tutorial?
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