How to assign English Alphabets to the characters of Encryption Key while avoiding hard coding?

In this problem of decryption of a message, I have to assign each Encryption Key character to an English alphabet without hard coding with the following criteria: A key would have at least one instance of each letter in the alphabet(without whitespaces!) I tried to do it by creating a map but with the repetition of a character(not to be included again) in key the actual alphabet also gets move ahead. How can it be achieved?

key = "the quick brown fox jumps over the lazy dog"

var decodeMessage = function (key, message) {
  let sub = new Map();
  key = key.replace(/\s/g, "");
  for (let i = 0; i < key.length; i++) {
    if (sub.has(key[i])) {
      continue;
    } else {
      sub.set(key[i], String.fromCharCode(97 + i));
    }
  }
  return sub;
};


Output:
Map {
  't' => 'a',
  'h' => 'b',
  'e' => 'c',
  'q' => 'd',
  'u' => 'e',
  'i' => 'f',
  'c' => 'g',
  'k' => 'h',
  'b' => 'i',
  'r' => 'j',
  'o' => 'k',
  'w' => 'l',
  'n' => 'm',
  'f' => 'n',
  'x' => 'p',
  'j' => 'q',
  'm' => 's',
  'p' => 't',
  's' => 'u',
  'v' => 'w',
  'l' => '}',
  'a' => '~',
  'z' => '',
  'y' => '€',
  'd' => '',
  'g' => 'ƒ' }