Complete the solution so that it splits the string into pairs of two characters. If the string contains an odd number of characters then it should replace the missing second character of the final pair with an underscore ('_').

Split Strings

function solution(str){
  var str_array = str.split('');
  var result = [];
  str_array.forEach((value, key) => {
    if(key % 2 == 0) {
      let next = (str_array[key + 1] ? str_array[key + 1] : '_')
      result.push(value + next);
    }
  }); 
  return result;
}