In JavaScript, functions are first-class objects

// Function declaration
function greet(name) {
    return "Hello, " + name + "!";
}

// Assigning a function to a variable
var sayHello = greet;

// Using the function through the variable
console.log(sayHello("Tom")); // Output: Hello, Tom!

// Passing a function as an argument to another function
function executeFunction(func) {
    return func("Jerry");
}

console.log(executeFunction(greet)); // Output: Hello, Jerry!

// Returning a function from another function
function createGreetingFunction() {
    return function(name) {
        return "Hola, " + name + "!";
    };
}

var spanishGreet = createGreetingFunction();
console.log(spanishGreet("Xavier")); // Output: Hola, Xavier!