typescript implementation of the supermemo 2 algorithm

sm2.ts

interface CardAnswer {
  [key: string]: (boolean | number | date)
  hesitationDetected: boolean;
  correct: boolean;
  timeSpent: number;
  date: date;
  ef?: number;
  interval?: number;
}

const calcInt = (rep: number, ef: number /* 1.3 <= easinessFactor <= 2.5 */): number => 
  rep === 1 ? 1
  : rep === 2 ? 6
  : calcInt(rep - 1, ef) * ef;

const deriveEf = (prevEf: number, q: 0|1|2|3|4|5): number => 
  Math.max(1.3, Math.min(2.5, 
        prevEf + (0.1 - (5 - q) * (0.08 + (5 - q) * 0.02))))

const calcEf = (as: CardAnswer[]) =>
  as.length > 1
  ? deriveEf(calcEf(as.slice(0, as.length - 1)), calcQual(as[as.length - 1]))
  : 2.5;

const calcQual = (a: CardAnswer) => 
  a.correct
  ? (!a.hesitationDetected && a.timeSpent < 5000
     ? 5
     : !a.hesitationDetected || a.timeSpent < 5000
     ? 4
     : 3)
  : (!a.hesitationDetected && a.timeSpent < 5000
     ? 2
     : !a.hesitationDetected || a.timeSpent < 5000
     ? 1
     : 0);

const answers: CardAnswer[] = [
      {
        "hesitationDetected": true,
        "correct": true,
        "timeSpent": 5000,
        "date": new Date("2018-11-01T15:13:00.000Z")
      },
      {
        "hesitationDetected": false,
        "correct": false,
        "timeSpent": 10000,
        "date": new Date("2018-11-02T06:30:00.000Z")
      },
      {
        "hesitationDetected": true,
        "correct": true,
        "timeSpent": 1000,
        "date": new Date("2018-11-02T07:01:00.000Z")
      }
    ];

console.log(
  answers
    .reduce((as, a, i) => [...as, {...a, ef: calcEf([...as, a]) }], [])
    .map((answer: CardAnswer, i: number) => ({ ...answer, interval: calcInt(i + 1, answer.ef)}))
    .map(({ef, interval}) => ({ef, interval}))
);