dropRepeats operator for mithril stream

mithril-stream-drop-repeats.js

/**
 * Creates a dependent stream that only updates when the source stream value differs from the previous
 * @see https://github.com/spacejack/mithril-stream-extra
 * @param function a custom function to validate equals, default is '===' check
 * @returns The resulting dependent stream
 * @author Jesus E. Franco Martinez <tezcatl@fedoraproject.org> 2019
 */
stream.dropRepeats = function dropRepeats (customEquality) {
  var oldVal // undefined for start
  if (typeof customEquality !== 'function') {
    customEquality = function (oldVal, newVal) {
      return oldVal === newVal
    }
  }
  return stream.map(function (newVal) {
    if (customEquality(oldVal, newVal)) {
      return Stream.SKIP
    }
    oldVal = newVal
    return newVal
  })
}
// this should be pushed as PR to mithril streams eventually