distinct.js

const identity = x => x

/**
 * Creates a predicate function to check if a value is not seen before
 *
 * @param {function} [by = x=>x]
 * @returns {function} predicate function
 *
 * @example
 *
 *  const uniqueUsers = users.filter(distinct(it => it.id))
 */
function distinct (fn = identity) {
  const seen = new Set()
  return x => seen.size < seen.add(fn(x)).size
}