reduce() method on an array produces a single value by applying a reducer function to every element of the input array. It takes a callback function with an input array, an accumulator (it is maintained throughout the loop), current value, and index.

reduce() is helpful in situations where you need to do some work on every element of the array and produce one result. Maybe you need to add every element of an array.

const arr = [2, 4, 9, 22];
const reduce = arr.reduce((final, current) => final + current);
console.log(reduce); // 37 (2 + 4 + 9 + 22)

Here final is the accumulator in which we keep adding all the values of the original input array. current denotes the current value of the loop.

You can also access the current index of the array using a map. The callback function takes a second argument for index.

const arr = [2, 4, 9, 22];
// we will add all numbers as well as their indices
const reduce = arr.reduce((final, current, index) => final + current + index);
console.log(reduce); // 43 (2 + 0 + 4 + 1 + 9 + 2 + 22 + 3)

And if access to index is not enough, you can also get access to the original array as a fourth parameter.

🚨 Unlike map(), reduce() does not change the original array.

We will go over one more example to understand the power of reduce and the situations it could be used in. We will flatten an array without the use of flat method.

const flatArray = (arr) => {
    let output = [];
    return arr.reduce((final, value) => {
        // use recursion, if value then concat to our final array else call flatArray again with the child array
        return final.concat(Array.isArray(value) ? flatArray(value) : value);
    }, output);
}

const input = [1, 2, 3, [10, 11, 12], 21, 22, 23, [31, 32, 33, 34], [41, 42]];
console.log(flatArray(input)); // [ 1, 2, 3, 10, 11, 12, 21, 22, 23, 31, 32, 33, 34, 41, 42 ]

🚨 reduce has a second parameter, and it is not the this context like a map; instead, it is the initial value to start with!

Look at the example above and see that we have provided the initial value as output. If no value is provided, it takes the first element of the array as the initial value, technically starting the loop from the second value of an array.