findmissing.js

function findMissingValue(values) {
  let seq = values.reduce((result, value, index) => {
    let nextIndex = index + 1;

    if (nextIndex < values.length) {
      let deference = values[nextIndex] - value;
      result.push(deference);
    }

    return result;
  }, []);

  let outcome = [];
  let sequence = seq.filter((item, i, ar) => ar.indexOf(item) !== i)[0];
  let maxCount = values[values.length - 1];

  for (let i = values[0]; i <= maxCount; i = i + sequence) {
    outcome.push(i);
  }

  return outcome.filter((i) => !values.includes(i));
}

let [missing] = findMissingValue([1, 3, 4, 5, 6]);
console.log(missing);