What is a function in JavaScript?

A function in JavaScript is a block of reusable code that performs a specific task. Functions can accept input data (arguments) and return a value.

Types of functions:

• Function Declaration:

  // Declaration of function 'greet'
  function greet(name) {
    return "Hello, " + name + "!";
  }
  

• Function Expression:

  // Function expression assigned to variable 'sayHello'
  const sayHello = function(name) {
    return "Hello, " + name + "!";
  };
  

• Arrow Function:

  // Arrow function for concise syntax
  const multiply = (a, b) => a * b;
  

Main features:

• First-Class Objects: Functions can be assigned to variables, passed as arguments, and returned from other functions.
• Scope: Functions create their own variable scope (local variables).
• Closures: Functions can "remember" and access variables from their outer scope even after that outer function has finished execution.

Example of closure:

function createCounter() {
  let count = 0; // Variable in outer scope

  return function() {
    count++;
    return count;
  };
}

const counter = createCounter();
console.log(counter()); // Output: 1
console.log(counter()); // Output: 2