How to sort two sorted arrays without using sort methods (sort) nor sort algorithms (bubble sort, quick sort)

I was asked in an interview and my answer was similar to this, which is wrong due to the final loop.

const newSortArrays = (arr1, arr2) => {
     let output = [];
     while (arr1.length && arr2.length) {
        if (arr1[0] < arr2[0]) 
         output.push(arr1[0] < arr2[0] ? arr1.shift() : arr2.shift())
     }
     return [...output, ...arr1, ...arr2]
 }

Answer

What you are talking about — “sorting” two arrays that are each themselves already sorted — is called a merge. This is how you do that:

function merge( left = [] , right = [] )  {
  const merged = new Array( left.length + right.length );

  let i = 0 ;
  let j = 0 ;
  let k = 0 ;

  // while both lists have items
  while ( i < left.length && j < right.length ) {
    const x = left[i];
    const y = right[j];

    if ( x <= y ) {
      merged[k++] = x;
      ++i;
    } else {
      merged[k++] = y;
      ++j;
    }

  }

  // if the left list still has items, take them
  while ( i < left.length ) {
    merged[k++] = left[ i++ ];
  }

  // if the right list still has items, take them
  while ( j < right.length ) {
    merged[k++] = right[ j++ ];
  }

  return merged;
}