Promise-like JS microlib

J.js

const PENDING = -1
const SUCCESS = 0
const ERROR = 1

class J {
  constructor (fn) {
    this.value = null
    this.state = PENDING
    this.successReactions = []
    this.failureReactions = []
    this.completeReactions = []

    fn(
      this.__makeCallback(SUCCESS),
      this.__makeCallback(ERROR)
    )
  }

  __makeCallback = state => value => {
    if (this.state !== PENDING) return
    this.value = value
    this.state = state
    this.__emit()
  }

  __react = callbacks => {
    while (callbacks.length > 0) callbacks.shift()(this.value)
    while (this.completeReactions.length > 0) this.completeReactions.shift()()
  }

  __emit = () => {
    switch (this.state) {
      case SUCCESS:
        this.__react(this.successReactions)
        break
      case ERROR:
        this.__react(this.failureReactions)
        break
    }
  }

  onSuccess = fn => {
    this.successReactions.push(fn)
    this.__emit()
  }

  onFailure = fn => {
    this.failureReactions.push(fn)
    this.__emit()
  }

  onComplete = fn => {
    this.completeReactions.push(fn)
    this.__emit()
  }
}