check if a number belongs to an AP described by first, last term and the diff

belongsToAP.js

const belongsToAP = (first, diff, last) => {
  return num => {
    if (num < first || num > last) {
      return false
    }
    const res = (num - first) / diff + 1
    if (Number.isInteger(res)) {
      return true
    }
    return false
  }
}