function sayHello(){
     console.log("Hello")
}
sayHello()   // Hello

1. Storing in a variable: Functions can be stored in a variable.

let func = function() {
    return "Hello!";
};

2. Returning as a result: Functions can be returned as a result from other functions.

function returnFunc() {
    return function() {
        return "Returned function!";
    };
}

let newFunc = returnFunc();
console.log(newFunc());

In our code, a function is returned within returnFunc, and we assigned it to a variable named newFunc by calling returnFunc(). Notice that () is used to invoke a function. In the last line, we call the returned function again using the newFunc() variable.
You can also call the returned function directly without assigning it to a variable by using double parentheses like returnFunc()().

3. Passing as an argument: Functions can be passed as arguments to other functions.

function sayHello() {
    return "Hello ";
}

function greet(hiFunc, name){
		console.log(hiFunc() + name);
}

greet(sayHello, "Ali");

As you can see in the code, the sayHello() function returns the word "Hello".
The greet() function accepts hiFunc as a function for the first argument and name as a string for the second argument.
Notice that we passed the sayHello() function as the first argument to the greet() function in the last line, which is exactly the scenario where the First class function concept applies. The second argument is a string.
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Be careful when working with these () parentheses. If you call the function with parentheses like greet(sayHello(), "Ali"); it will cause an error because the returned internal function won't be invoked!