kefir/src/one-source/diff.js

Reactive Programming library for JavaScript inspired by Bacon.js and RxJS with focus on high performance and low memory usage

import {createStream, createProperty} from '../patterns/one-source'
import {NOTHING} from '../constants'

const mixin = {
  _init({fn, seed}) {
    this._fn = fn
    this._prev = seed
  },

  _free() {
    this._prev = null
    this._fn = null
  },

  _handleValue(x) {
    if (this._prev !== NOTHING) {
      const fn = this._fn
      this._emitValue(fn(this._prev, x))
    }
    this._prev = x
  },
}

const S = createStream('diff', mixin)
const P = createProperty('diff', mixin)

function defaultFn(a, b) {
  return [a, b]
}

export default function diff(obs, fn, seed = NOTHING) {
  return new (obs._ofSameType(S, P))(obs, {fn: fn || defaultFn, seed})
}