test.js

function flatten(arr) {
  var flattenedArr = [];
  arr.forEach(function(nestedArr) {
    // if the array has nested elements, proceed
    if (nestedArr.length > 0) {
      nestedArr.forEach(function(elem) {
        // check if elem is already present in flattenedArr
        if (!testNumberAndNumberAsString(elem, flattenedArr)) {
          // if not, push it to the flattened array
          flattenedArr.push(elem);
        }
      });
    // if there are no nested elements, then return the input array
    } else {
      flattenedArr = arr;
    }
    
  });
  return flattenedArr;
}

function testNumberAndNumberAsString(input, arr) {
  var str = String(input);
  var num = Number(input);
  return (arr.includes(str) || arr.includes(num));
}

console.log(flatten([[1, '2', 3], [2, 3], [4, 5, 6], [5]]));
console.log(flatten([['a', 'b', 'c', 3], [3, 5]]));
console.log(flatten([]));
console.log(flatten([1]));
console.log(flatten([1, 2]));