Javascript Interview Prep: implement Array.prototype.flat()

Prompt

There is already Array.prototype.flat() in JavaScript (ES2019), which reduces the nesting of Array.

Could you manage to implement your own one?

Here is an example to illustrate

const arr = [1, [2], [3, [4]]];

flat(arr)
// [1, 2, 3, [4]]
flat(arr, 1)
// [1, 2, 3, [4]]
flat(arr, 2)
// [1, 2, 3, 4]

Answer

Here is an example I wrote of how to implement the flat() method. I used a recursive method to flatten the array.

Solution 1:

// This is a JavaScript coding problem from BFE.dev 
/**
 * @param { Array } arr
 * @param { number } depth
 * @returns { Array }
 */
function flat(arr, depth = 1) {
  const res = [];
  arr.filter((each, eachIndex) =>{
    // console.log(typeof each);
    if (Array.isArray(each) && depth > 0){
      const flattened = flat(each, depth-1);
      flattened.filter((each1, each1Index) => {
        res.push(each1);
      })
      
    } else {
      res.push(each)
    }
  });
  console.log(res.toString());
  return res;
}


const arr = [1, [2], [3, [4]]];

flat(arr, 1);

Solution 2:

function flat(arr, depth = 1) {
  if (depth === 0) return arr;
  const output = []
  for (const item of arr) {
    if (Array.isArray(item)) {
      output.push(...flat(item, depth - 1))
    } else {
      output.push(item);
    }
  }
  return output;
}


const arr = [1, [2], [3, [4]]];

console.log(flat(arr, 1));