Believe it or not, reduce can do the job of not only Map, filter but also includes, find, every, concat, and many other array functions, it can create objects, it can keep the track of count while iterating, it can filter out elements, and what not. Reduce is basically a super power of JS (one of many) and that’s exactly what we are going to see today! After learning how it works, we will implement some famous utility functions with Reduce. Maybe that’s for second part.

Why do I need it?

(Skip to Here if you already know what it does). Before diving into how it works, let’s understand what it does and Why do I need it! Let’s take an example of Map. the Map function iterates over an array and apply a given function to EACH element of the array. simple as that. give it something, it will apply it over each element.

const array = [2, 3, 4];
const incByOne = i => i+1;
const resArray =;   //=> [3, 4, 5]

now Reduce is somewhat very similar. you give it some function, it iterates over the array it is being applied on, and executes the given function over EACH element of the array. BUT! there’s a catch here, While iterating, the Reduce ‘Remembers’ the result of executing the function on previous element. and then apply the function on current element with the previous result and so on. don’t worry if you don’t understand anything, it WILL get clear believe me. for now, just understand that Reduce takes and array, reduces it to some value (usually, but not necessarily, a single element) here is an example. just look at the result for now, we will see how we got the result later.

const array = [2, 3, 4];
const reducer = (acc, curr) => acc + curr;
const resArray = array.reduce(reducer);   //=> 9 (addition of all the elements in array)

How it works?

Now let’s look at how we did it, and that’s is the most confusing part. Believe it or not, it may look weird at the start, but once you get a hang of it, you will start ‘seeing’ the magic behind it. Imagining how some code will flow through Reduce is not really that hard with some practice. I came across some tutorials on the Internet with the same topic “How reduce works”. Frankly one of them was literally incorrect (technically). So, we are going to dive into our own tutorial of how it works!

The first thing to understand is how many parameters does it take. Like Map it takes a callback function, plus a starting point. the callback function (reducer function or just reducer) is mandatory, starting point is optional.

const reduced = array.reduce(reducer, init);

the reducer takes 4 parameters:

  • Accumulator acc
  • Current Value curr
  • Current Index id
  • Source Array arr

Out of these four the first two are the ones you will need most of the time. at least for learning. I came across very limited usecases where you need the third param i.e. id and no use case where you need the last param i.e. arr. So just for the information, I am keeping it there, some of you might need it somewhere.

the next part is to understand what the heck this Accumulator is? consider it as a storing space. When I said, the Reduce ‘Remembers’ the result, I meant when the function is fired on ith element, the output is stored in this first variable called acc. then reducer moves on to i+1st element, takes the previous result (acc), takes current element (curr), executes the function on both of these (acc+curr in our example), and again stores the result in acc and moves on to i+3rd element. and so on until array is exhausted. Still confused? it’s time to be a debugger ourself. in the above example, the array is [2, 3, 4] and reducer function says (acc, curr) => acc + curr. For those who are not confortable with fat arrow notation, it is same as

function reducer(acc, curr) {
  return acc + curr;
// OR
var reducer = function(acc, curr){
  return acc + curr;
pick whatever you like, just be syntactically correct 😄. Now, we are going to Reduce the given array step by step.

step 1:

as initial point init is not given, the reducer picks 2 element from array, 0th, and 1st, assign 0th to the acc and 1st to the curr part. thus

acc = 2;
curr = 3;
now our function says return acc+curr; easy!. 2 + 3 = 5 1st grade maths right? as I said, this result of addition is stored in acc. So, now:
acc = 5;
curr = 4;
note that we have reached to the end of array, thus, there will not be any element to assign for curr in next iteration. so this is our last operation. again, as said, the reducer returns acc + curr so 5 + 4 = 9 is returned, and stored in acc again. Because there is nothing to iterate on for next loop, the result stored in acc is returned. Thus we get 9, which is the result of addition of all elements. before trying one more very similar example, go ahead and try this addition of all elements on some different arrat and check if the results are correct.

Now, we will write a function for finding factorial of a number with reduce. 😁. don’t worry it’s nothing different at all than previous function! infact, we will just change one single operator and we will have what we desire. the problem here is, array needs to start from 1, 2, ... and so on for factorial thing. otherwise it will act as a ‘multiplication of all the elements in array’ (What else is a factorial )

let’s find the factorial of ‘5’ with reduce. which is 120 we already know it.

const factorialFn = (acc, curr, id) => acc * curr;
const fiveFactorial = [1,2,3,4,5].reduce(factorialFn);   //=> 120
Yup! that was it. now let’s debug it, this time we will keep the track of id as well, but we won’t use it. Now, we know that id is gonna vary from 0 to 4. for first iteration, as we have not given anything for initial point, acc will hold the value of 0th index of array and current will hold the value of 1st index. thus we have,
id = 0;
acc = 1;
curr = 2;
now, the reducer says, take these two, multiply them, and return them. So, 1 * 2 = 2. Thus, for id = 1 the acc now holds 3 and curr holds 3.
id = 1;
acc = 2;
curr = 3;
multiplication of these two: 2 * 3 = 6. now 6 is stored in acc and reduce moves on to next index. thus we have
id = 2;
acc = 6;
curr = 4
And this repeats until there are no elements left in the array. So for final iteration we will have
acc = 24;
curr = 5;
thus, 24 * 5 = 120 Voila! we have our factorial reduced from Array!

This is just the starting. But I think the post is getting a little bit longer, so I will stop (with this one) here, and continue the best and most important part in the next tutorial. Until then, 🙏