 # Counter Using Functional Approach in Javascript

In my previous post, I showed my readers how to implement Singleton in JavaScript using closure, and IIFE.

This time, I want to show you how to utilize the same building blocks, as well as one more functional programming technique, to implement a basic counter!

Let's start with a counter that takes a starting number as an argument, and uses closure to keep track of the current count:

function from(start) {
let i = start - 1

function inc() {
i = i + 1
return i
}
return inc
}


As you can see, I have outer function from() which takes parameter start. Then, I initialize i with the value of start - 1, and enclose it in inner function inc(). Every time inc() is executed, it increases i, and returns it. Finally, the inner function is returned from the outer.

Now, let's see how to use this counter:

(function () {
let count = from(0)

for(let i = 0; i < 10; i++) {
console.log(
count()
)
}
}())


I wrapped the counter in anonymous IIFE, just because it is a good practice to separate variables from global scope, even when doing something as simple as this example.

Notice, how closure allows this counter to have a "memory". Similar technique can be utilized in order to implement memoization in more advanced and computationally heavy algorithms.

Another thing I want to show you is a concept of higher-order functions. It is exactly what it sounds like:

In mathematics and computer science, a higher-order function is a function that does at least one of the following: takes one or more functions as arguments, returns a function as its result.

Actually, if you think about it, our function from already fall under definition of the higher-order function. It returns another function, inc. Let's make something that satisfy both properties of the definition!

I will build a counter that starts counting from a given value, but doesn't count past certain limit. Past the limit, it returns undefined. I have the counting logic written in the example above, and only need to handle the limit part. A good approach is to create a function to that takes two arguments: a counter, and a limit. It then returns another function, that calls a counter, and makes sure the limit is not reached. Here is an implementation:

function to(counter, limit) {
return function() {
let j = counter();
if(j > limit) {
return undefined
}
return j
}
}


Notice, that in the example I am returning an anonymous function. I actually did it on purpose, in order to show a reader that JavaScript is pretty flexible in this extend. You can as well use an arrow function. It is all up to your particular implementation!

Finally, I will include an example of the whole program:

function from(start) {
let i = start - 1

function inc() {
i = i + 1
return i
}
return inc
}

function to(counter, limit) {
return function() {
let j = counter();
if(j > limit) {
return undefined
}
return j
}
}

(function (){
let count = to(from(3), 10);

for(let i = 0; i < 10; i++) {
console.log(
count()
)
}
}())


To sum up, I showed a reader how to use closure in order to implement a simple counter, and introduced a notion of higher-order function. Also, I gave a hint on how to implement memoization using approach from this example! If you have any questions, let me know in comments!

Happy hacking!

### Discussion   