function.md

JavaScript Functions

❓ What is a Function?

A function is a set of instructions written once and used many times.

function sayHi() {
  console.log("Hi!");
}

sayHi(); // Hi!
sayHi(); // Hi! (use it again!)

---

📋 Ways to Write a Function

1. Function Declaration

function func_name(params) { statements }

Example

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

2. Function Expression

const func_name = function(params) { statements };

Example

const greet = function(name) {
  console.log("Hi, " + name + "!");
};

3. Arrow Function (Modern & Short)

const func_name = (params) => { statements };

Example

const greet = (name) => { console.log("Hi, " + name + "!"); };

Tip

1. function keyword
2. function and const keywords
3. const keyword

---

📥 Parameters & Arguments

Parameter = the placeholder in the definition of the function
Argument = the real value passed into the function when you call it

Example

//                👇 parameters
function add(num1, num2) {
  console.log(num1 + num2);
}

//     👇 arguments
add(10, 5); // 15
add(x, y);  // x + y

---

↩️ Return statement

Use return when you want the function to give you a result.

Example

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

const answer = multiply(4, 5);
console.log(answer); // 20

Important

After return, the function stops. Nothing below it runs.

Example

function check(age) {
  if (age >= 18) return "Adult";
  return "Minor";
  console.log("Thank you!"); // Never executes
}

console.log(check(20)); // Adult
console.log(check(15)); // Minor

---

🏷️ Default Parameters

Default parameters are set to maintain the function flow incase of no arguemnets being passed in the function call.

Example

function welcome(name = "Guest") { // Take value of name as "Guest" whenever no arguements in function call 
  console.log("Welcome, " + name + "!");
}

welcome("Priya"); // Welcome, Priya!
// When no arguements are passed
welcome();         // Welcome, Guest!

---

🚧 Variable scope

Scope of a variable is the region of the code where it is accessible.
When variables are declared inside a function, they are called as local variables (function variables) and cannot be accessed outside of the function.

Example

function myFunc() {
  const secret = "I'm inside!";
  console.log(secret); // Works
}

myFunc();
console.log(secret); // secret doesn't exist here

---

🔁 Callbacks

Function callback is used when you want some set of functions to be executed in order without fail.

Example

const sum = (callback, x, y) => {
  let result = 0;  
  setTimeout(() => { result = x + y: }, 3000); // wait for 3s (3000 ms) and then execute { result = x + y; }
  callback(result);
}
const display(result){
  console.log(result);
}

// You want the functions to execute in the order sum() --> display() strictly
// Hence send display as a callback into sum() asking it to execute display() once itself is completely executed.

sum(display, 12, 18); // 18

// No need to write display function separately.

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