Simulation of flat

flat.js

const testArray = [1, [3, [4]], [3, [4, [5]]], [2, [3, [4, [5]]]], [2, [3, [4, [5]]]]];

function flatten(arr, depth = 1) {
  if (depth <= 0) return arr;

  return arr.flatMap((item) => {
    if (Array.isArray(item) && item.some(Array.isArray)) {
      return flatten(item, depth - 1);
    }
    return item;
  });
}

function flatten(arr, depth = 1) {
  if (depth <= 0) return arr;

  return arr.reduce((previousValue, currentValue) => {
    // Option 1
    return previousValue.concat(
      Array.isArray(currentValue) ? flatten(currentValue, depth - 1) : currentValue,
    );
    // Option 2
    // if (Array.isArray(currentValue)) {
    //   return [...previousValue, ...flatten(currentValue, depth - 1)];
    // }
    // return [...previousValue, currentValue];
  }, []);
}

function flatten(arr, depth = 1) {
  if (depth <= 0) return arr;

  let result = [];

  for (const item of arr) {
    if (Array.isArray(item)) {
      // Option 1
      result = result.concat(flatten(item, depth - 1));
      // Option 2
      // result.push(...flatten(item, depth - 1));
    } else {
      result.push(item);
    }
  }

  return result;
}

function flatten(arr, depth = 1) {
  while (depth > 0 && arr.some((item) => Array.isArray(item))) {
    depth -= 1;
    // Option 1
    arr = [].concat(...arr);
    // Option 2
    // arr = Array.prototype.concat.call([], ...arr);
    // Option 3
    // arr = Array.prototype.concat.apply([], arr);
  }
  return arr;
}

console.log('flatten(testArray, 3): ', flatten(testArray, 3));
console.log('testArray.flat(3): ', testArray.flat(3));