whats difference here with string and numbers in javascript arrays?

Question

I am merging two sorted arrays in JavaScript. When I call function with two arrays having numbers it works fine, but when I call that function with strings then it does not work. Why?

function mergeSortedArrays(array1, array2) {
  const mergedArray = [];
  let array1Item = array1[0];
  let array2Item = array2[0];
  let i = 1;
  let j = 1;

  if (array1.length === 0) {
    return array2;
  }
  if (array2.length === 0) {
    return array1;
  }

  while (array1Item || array2Item) {
    if (array2Item === undefined || array1Item < array2Item) {
      mergedArray.push(array1Item);
      array1Item = array1[i];
      i++;
    } else {
      mergedArray.push(array2Item);
      array2Item = array2[j];
      j++;
    }
  }
  console.log(mergedArray);
}

//working?
mergeSortedArrays([0, 3, 4, 12, 222], [3, 4, 6, 30]);

// not working why?
mergeSortedArrays(["0", "3", "4", "12", "222"], ["3", "4", "6", "30"]);

Answer 1

As said in the comments, strings in JS are compared lexically , so, "222" is smaller than "3".

A solution that I see that you can use, is this one:
After checking the arrays for nullity, then concat then into mergedArray, then use the JS function sort(), with the basic return of value1 - value2, that way it will sort the strings in the order you want, and also will work for numbers.

(Further read: Why is one string greater than the other when comparing strings in JavaScript?)

function mergeSortedArrays(array1, array2) {
  let mergedArray = [];

  if (array1.length === 0) {
    return array2;
  }
  if (array2.length === 0) {
    return array1;
  }

  mergedArray = array1.concat(array2)
  mergedArray.sort(function(a, b) {
    return a - b
  })
  console.log(mergedArray);
  return mergedArray;
}


mergeSortedArrays([0, 3, 4, 12, 222], [3, 4, 6, 30]);
mergeSortedArrays(["0", "3", "4", "12", "222"], ["3", "4", "6", "30"]);

BUT, be knowing that this solution will only work as expected if the strings are representations of numbers ("1", "2",...), if it is something like "aa", "abc", "b" it probably won't work well and another solution may be needed. (something like this: https://stackoverflow.com/a/51169/8732818)

Answer 2

Sort for string works differently than numbers. Its based on the ASCII table values. For example "99" > "100000" return should return true