flattenArray.js

const input = [
  1, 2, 3,
   [4],
  [5, 6, [7], [8, [9, [10]]]],
  11, 12, 13,
  [14, [[[[[15, [16]]]]]]],
  17, 18,
  [19, [20, [21, [22, [23, [24, [[[[[25]]]]]]]]]]]
 ];
 
// SOLUTION 1
const resultArry = [];
Array.prototype.flattenNaïve = function() {
const obj = this || [];
if(obj.forEach) {
  obj.forEach(item => {
    if(Array.isArray(item)) {
      item.flattenNaïve();
    } else {
      resultArry.push(item);
    }
  });
}
  return resultArry;
}

console.log(input.flattenNaïve());


// SOLUTION 2
Array.prototype.flattenWithReduce = function() {
	if (this.reduce) {
  	return this.reduce((acc, item) => {
      if (Array.isArray(item)) {
	return [...acc, ...item.flattenWithReduce()]
      } else {
        return [...acc, item];
      }
    }, []);
  }
}

console.log(input.flattenWithReduce());

But wouldn't pushing to acc in the reduce function make it an impure function, hence violating the functional programming paradigm. Can we instead do it this way:

Array.prototype.flattenWithReduce = function() {
	if (this.reduce) {
  	return this.reduce((acc, item) => {
      if (Array.isArray(item)) {
        return [...acc, ...item.flattenWithReduce()];
      }
      return [...acc, item];
    }, []);
  }
}

Just suggesting, I might be wrong as well 😅

Hey Priyadarshanvijay, completely agree with what you pointed out here. Pure functions are indeed the essence of FP. Pushed another commit with the changes

Thanks @rajatjain-21 , learnt all this from CodeAsylums only 😄