chord-progressions.js

const pickOne = arr =>
  arr.length > 1 ? arr[Math.round(Math.random())] : arr[0];
const dice = () => !!Math.round(Math.random());

const getProgFactory = ({ T, P, D }) => {
  return (count = 4) => {
    const chords = [];

    // Push root/tonic
    chords.push(pickOne(T));

    let i = 1;

    // Pick a predominant
    if (i < count - 1) {
      chords.push(pickOne(P));
      i++;
    }

    // Try another predominant
    if (i < count - 1 && dice()) {
      chords.push(pickOne(P));
      i++;
    }

    /////////4 or more//////////
    if (i < count - 1) {
      // Pick a dominant
      chords.push(pickOne(D));
      i++;
    }

    if (i < count - 1) {
      // Pick a predominant
      chords.push(pickOne(P));
      i++;
    }

    if (i < count - 1) {
      // Pick a dominant
      chords.push(pickOne(D));
      i++;
    }

    // Pick a predominant if possible
    if (i < count - 1 && dice()) {
      chords.push(pickOne(P));
      i++;
    }
    ////////////////////////////

    // Fill the rest with dominant
    while (i < count) {
      chords.push(pickOne(D));
      i++;
    }

    return chords;
  };
};

const M = getProgFactory({ T: ['I', 'vi'], P: ['ii', 'IV'], D: ['V'] });
const m = getProgFactory({ T: ['i', 'VI'], P: ['ii', 'iv'], D: ['V'] });

console.log(M(4));
console.log(M(8));
console.log(m(4));
console.log(m(8));