JavaScript

In JavaScript, a function is a reusable block of code designed to perform a specific task. Defining a function means creating a set of instructions that can be called (or invoked) whenever needed.

There are multiple ways to define functions in JavaScript, each with its own syntax and behavior.

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1. Function Declaration (Named Function)

This is the most traditional way to define a function using the function keyword.

Syntax:

function greet(name) {
  return `Hello, ${name}!`;
}

Example:

console.log(greet("Alice")); // Hello, Alice!

Key Features:

• Hoisted (you can call the function before its declaration in the code)
• Useful for defining reusable logic

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2. Function Expression

A function can also be defined and stored in a variable.

Syntax:

const greet = function(name) {
  return `Hi, ${name}!`;
};

Example:

console.log(greet("Bob")); // Hi, Bob!

Note:

• Not hoisted (you must define before using)
• Can be anonymous (no name required inside function)

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3. Arrow Function (ES6+)

A cleaner and shorter syntax introduced in ES6, often used for callbacks and concise operations.

Syntax:

const greet = (name) => `Hey, ${name}!`;

Example:

console.log(greet("Charlie")); // Hey, Charlie!

Key Features:

• Shorter syntax
• No this, arguments, or super bindings (lexical this)
• Great for functional programming, but not suited for methods in objects or constructors

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4. Function Constructor (Less Common)

You can also define a function using the Function constructor.

Syntax:

const sum = new Function('a', 'b', 'return a + b');

Not Recommended:

• Slower
• Less readable
• Used in dynamic or advanced meta-programming

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5. Immediately Invoked Function Expression (IIFE)

Used to define and run a function immediately, without calling it later.

Syntax:

(function() {
  console.log("This runs instantly!");
})();

Useful for:

• Creating private scopes
• Avoiding global variable pollution

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Bonus: Method Definitions in Objects

You can define functions inside objects, known as methods.

const user = {
  name: "Devin",
  greet() {
    return `Hi, I'm ${this.name}`;
  }
};

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Function Hoisting Difference

greet(); //  Works
function greet() {
  console.log("Hello!");
}

greetAgain(); //  Error
const greetAgain = function () {
  console.log("Hi!");
};

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Summary Table

Method	Syntax Format	Hoisted	Uses this	Arrow Syntax
Function Declared	function name() {}		Yes
Function Expression	const fn = function(){}		Yes
Arrow Function	const fn = () => {}		No (lexical)
IIFE	(function() {})()		Yes

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