For a simple expression, you can make it even shorter:

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

This is called an implicit return.

🔟 Arrow Function Examples

One parameter:

const square = number => number * number;
console.log(square(5)); // 25

Multiple parameters:

const multiply = (a, b) => a * b;
console.log(multiply(4, 5)); // 20

No parameters:

const greet = () => {
    console.log("Hello");
};
greet();

1️⃣1️⃣ Default Parameters

You can provide a default value for a parameter.

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

greet("Rahul"); // Hello Rahul

greet(); // Hello Guest

Default parameters are very useful when a value is optional.

1️⃣2️⃣ Rest Parameters

Sometimes you don't know how many arguments will be passed.

You can use "...".

function add(...numbers) {
    console.log(numbers);
}

add(10, 20, 30, 40); // [10, 20, 30, 40]

The rest parameter collects the arguments into an array.

function add(...numbers) {
    let total = 0;
    for (let number of numbers) {
        total += number;
    }
    return total;
}

console.log(add(10, 20, 30)); // 60

1️⃣3️⃣ Scope

Scope determines where a variable can be accessed.

function greet() {
    let message = "Hello";
    console.log(message);
}

greet();

"message" exists inside the function. Trying to access it outside: console.log(message); will cause an error.

This is because "message" has function/local scope.

1️⃣4️⃣ Block Scope

Variables declared with "let" and "const" are block-scoped.

if (true) {
    let message = "Hello";
    console.log(message);
}

This works. But: console.log(message); outside the block does not. The block is defined by { // block }.

This is one reason "let" and "const" are preferred over "var".

1️⃣5️⃣ Global Scope

A variable declared outside functions or blocks can be accessible from broader parts of the program.

const appName = "My App";

function showAppName() {
    console.log(appName);
}

showAppName(); // My App

However, don't create unnecessary global variables. Too many globals can make larger applications difficult to maintain.

⭐ 1️⃣6️⃣ Callback Functions

A function can be passed to another function.

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

function processUser(callback) {
    callback("Rahul");
}

processUser(greet); // Hello Rahul

Here: greet is passed as a value to processUser. This is called a callback function. Callbacks are extremely important in JavaScript because you'll encounter them throughout: Events, Array methods, Asynchronous programming, APIs, Node.js

⭐ 1️⃣7️⃣ Higher-Order Functions

A higher-order function is a function that: accepts another function as an argument, or returns a function.

function calculate(a, b, operation) {
    return operation(a, b);
}

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

console.log(calculate(10, 5, add)); // 15

Here "calculate()" receives another function. That's a higher-order function.

⭐ 1️⃣8️⃣ Functions Returning Functions

A function can also return another function.

function createGreeting(name) {
    return function () {
        console.log("Hello " + name);
    };
}

const greetRahul = createGreeting("Rahul");
greetRahul(); // Hello Rahul