To run: "node getLongestSubstring.js <string>". Get longest substring in alphabetical order from a string. Javascript-specific methods were avoided as much as possible. Complexity is O(n).

getLongestSubstring.js

/*
 * Assume s is a string of lower case characters.
 *
 * Write a program that prints the longest substring of s
 * in which the letters occur in alphabetical order.
 *
 * For example, if s = 'azcbobobegghakl', then your program should print
 * "Longest substring in alphabetical order is: beggh"
 *
 * In the case of ties, print the first substring.
 * For example, if s = 'abcbcd', then your program should print
 * "Longest substring in alphabetical order is: abc"
 */ 
 
/* Just a switch() statement. */
function charToNum(char) {
  switch(char){
    case 'a':
      return 1;
      break;
    case 'b':
      return 2;
      break;
    case 'c':
      return 3;
      break;
    case 'd':
      return 4;
      break;
    case 'e':
      return 5;
      break;
    case 'f':
      return 6;
      break;
    case 'g':
      return 7;
      break;
    case 'h':
      return 8;
      break;
    case 'i':
      return 9;
      break;
    case 'j':
      return 10;
      break;
    case 'k':
      return 11;
      break;
    case 'l':
      return 12;
      break;
    case 'm':
      return 13;
      break;
    case 'n':
      return 14;
      break;
    case 'o':
      return 15;
      break;
    case 'p':
      return 16;
      break;
    case 'q':
      return 17;
      break;
    case 'r':
      return 18;
      break;
    case 's':
      return 19;
      break;
    case 't':
      return 20;
      break;
    case 'u':
      return 21;
      break;
    case 'v':
      return 22;
      break;
    case 'w':
      return 23;
      break;
    case 'x':
      return 24;
      break;
    case 'y':
      return 25;
      break;
    case 'z':
      return 26;
      break;
    default:
      return 0;
      break;
  }
}

/* Iterates through the entire string input of length n. */
function stringToNumArr(str) {
  var tempArr = [];

  /* Convert each character to the respective number. */
  for (let i = 0; i < str.length; i++) {
    tempArr[i] = charToNum(str[i]);
  }
  return tempArr;
}

/* Just a switch() statement. */
function NumToChar(num) {
  switch(num){
    case 1:
      return 'a';
      break;
    case 2:
      return 'b';
      break;
    case 3:
      return 'c';
      break;
    case 4:
      return 'd';
      break;
    case 5:
      return 'e';
      break;
    case 6:
      return 'f';
      break;
    case 7:
      return 'g';
      break;
    case 8:
      return 'h';
      break;
    case 9:
      return 'i';
      break;
    case 10:
      return 'j';
      break;
    case 11:
      return 'k';
      break;
    case 12:
      return 'l';
      break;
    case 13:
      return 'm';
      break;
    case 14:
      return 'n';
      break;
    case 15:
      return 'o';
      break;
    case 16:
      return 'p';
      break;
    case 17:
      return 'q';
      break;
    case 18:
      return 'r';
      break;
    case 19:
      return 's';
      break;
    case 20:
      return 't';
      break;
    case 21:
      return 'u';
      break;
    case 22:
      return 'v';
      break;
    case 23:
      return 'w';
      break;
    case 24:
      return 'x';
      break;
    case 25:
      return 'y';
      break;
    case 26:
      return 'z';
      break;
    default:
      return '_';
      break;
  }
}

/* Iterates through the substring (with length no longer than n). */
function NumArrToString(arr) {
  var tempArr = [];
  var stringified = '';

  /* Convert each number to the respective character. */
  for (let i = 0; i < arr.length; i++) {
    tempArr[i] = NumToChar(arr[i]); 
  }

  /* This is the equivalent of arr.join(separator) in Javascript. */
  for (let i = 0; i < tempArr.length; i++) {
    stringified = stringified+tempArr[i];
  }

  return stringified;
}


function getLongestSubstring(str) {
  /* Convert the input string into an array of numbers. */
  var arr = stringToNumArr(str);

  var longestSubstring = [];
  var currentSubstring = [];

  /* Iterates through the array of numbers to look for longest substring. */
  for (let i = 0; i < arr.length; i++) {
    if(currentSubstring.length === 0) currentSubstring[i] = arr[i]; // If currentSubstring is empty, auto insert current character.
    else{
      /* If current character is greater than or equal to the last character in currentSubstring, insert it. */
      if(arr[i] >= currentSubstring[currentSubstring.length-1]) currentSubstring[currentSubstring.length] = arr[i];
      else{
        /* If current character is less than the last character in currentSubstring, reset currentSubstring's value with current character. */
        if(currentSubstring.length > longestSubstring.length) longestSubstring = currentSubstring; // If currentSubstring's length is greater than longestSubstring's length, update longestSubstring.
        currentSubstring = [arr[i]]; // This is where currentSubstring is reset.
      }
    }
  }

  return longestSubstring = NumArrToString(longestSubstring);
}

var substr = getLongestSubstring(process.argv[2]);

console.log(substr);