Making sense of Reduce in JS: Reduce is a higher-order function that is available as an Array method, just like Map or Filter, Let's take an example where we first declare an array that has some number values, and we want to print the sum of all the values in that array.

let numbers = [1,2,3,4,5];
let sum = 0;
for(let i = 0; i < numbers.length; i++) {
	sum += numbers[i];
}
console.log(sum); // 15

To understand reduce we would have to understand what is happening in the above code.

So far so good. Now think of the sum variable here as the accumulator, meaning it is eventually responsible to store the result of the for loop, and i in the for loop as the current value because i represent the current value like 1,2,3... as it iterates through the array.

Let's rewrite the above code but with reduce.
let numbers = [1,2,3,4,5];
const total = numbers.reduce((accumulator, currentValue) => {
	return accumulator + currentValue;
}, 0)
console.log(total); // 15

So, what is happening in the above code? If you look closely you will see that all we are doing here is calling the sum variable in this case calling it accumulator, inside our callback function in reduce rather than outside, currentvalue is equivalent to i in our previous example, so it iterates through the array numbers and adding it to the value. But what value? We did initiate the sum variable in the previous example with 0. We are doing the same thing here as well, the 0 after the curly braces is the initial value of the accumulator. So, lets write the code more descriptively

let numbers = [1,2,3,4,5];
const total = numbers.reduce((accumulator (or sum), currentValue (or i )) => {
	return accumulator + currentValue;
}, initial value of accumulator)
console.log(total); // 15

We can write less code as well:

let numbers = [1,2,3,4,5];
const total = numbers.reduce((acc, curr) => (acc + curr));
console.log(total) // 15

Thinking about reduce as an alternative to traditional for loop helped me wrap my head around it. Hopefully, it will help you as well.