interleave mutliple arrays in javascript

Question

We have an Array of arrays, which we want to interleave into a single array: i.e:

masterArray = [[1, 2, 3], ['c', 'd', 'e']] => [1, 'c', 2, 'd', 3, 'e'],

if arrays are not of equal length, pad it to the longest innerArray's length.

i.e [1, 2, 3], [4, 5]) => [1, 4, 2, 5, 3, null]

I've satisfied this condition with the case of 2 arrays, however if the case is more than that. I struggle to form a strategy on dealing with more than 2.

[1, 2, 3], [4, 5, 6], [7, 8, 9] => [1, 4, 7, 2, 5, 8, 3, 6, 9]

function interleave(...masterArray) {
  let rtnArray = [];
  let longestArrayPosition = getLongestArray(masterArray);
  let longestInnerArrayLength = masterArray[longestArrayPosition].length; 
  padAllArraysToSameLength(masterArray, longestInnerArrayLength); //pad uneven length arrays
  
  masterArray[0].forEach((firstArrayNum, index) => {
    const secondArrayNum = masterArray[1][index];
    rtnArray.push(firstArrayNum);
    rtnArray.push(secondArrayNum);
  });

  return rtnArray;
}

function getLongestArray(masterArray) {
  return masterArray
    .map(a=>a.length)
    .indexOf(Math.max(...masterArray.map(a=>a.length)));
}

function padAllArraysToSameLength(masterArray, maxLength) {
  return masterArray.forEach(arr => {
    if (arr != maxLength) {
      while(arr.length != maxLength) {
        arr.push(null);
      }
    }
  })
}

Answer 1

Use Array.from() to transpose the array of arrays (rows => columns and vice versa), and fill in the missing places with null. Flatten the tramsposed arrays of arrays with Array.flat():

const fn = arr => Array.from({ 
    length: Math.max(...arr.map(o => o.length)), // find the maximum length
  },
  (_, i) => arr.map(r => r[i] ?? null) // create a new row from all items in same column or substitute with null
).flat() // flatten the results

const arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

const result = fn(arr)

console.log(result)