Promise Implementation

promise.js

function addToTaskQueue(fn) {
  // 模拟浏览器微任务
  setTimeout(fn, 0)
}

function isThenable(value) {
  return typeof value?.then === 'function';
}

class MyPromise {
  #state
  #value
  #handlers

  constructor(fn) {
    this.#state = 'pending'
    this.#value = null
    this.#handlers = []
    this.#safeRun(fn, this.#resolve, this.#reject)
  }
  
  #safeRun = (fn, onFulfilled, onRejected) => {
    let done
    try {
      fn(
        (val) => {
          if (done) {
            return
          }
          done = true
          onFulfilled(val)
        },
        (val) => {
          if (done) {
            return
          }
          done = true
          onRejected(val)
        }
      )
    } catch (err) {
      if (done) {
        return
      }
      done = true
      onRejected(err)
    }
  }
  
  #resolve = (value) => {
    if (isThenable(value)) {
      this.#safeRun(value.then, this.#resolve, this.#reject)
    } else {
      this.#fulfill(value)
    }
  }

  #fulfill = (value) => {
    this.#state = 'fulfilled'
    this.#value = value
    this.#handlers.forEach(this.#handle)
    this.#handlers = null // garbage collecting
  }

  #reject = (err) => {
    this.#state = 'rejected'
    this.#value = err
    this.#handlers.forEach(this.#handle)
    this.#handlers = null // garbage collecting
  }

  #handle = (handler) => {
    if (this.#state === 'pending') {
      this.#handlers.push(handler)
    }
    if (this.#state === 'fulfilled') {
      handler.onFulfilled(this.#value)
    }
    if (this.#state === 'rejected') {
      handler.onRejected(this.#value)
    }
  }

  then = (onFulfilled, onRejected) => {
    const nextPromise = new MyPromise((resolve, reject) => {
      const fullfillmentTask = () => {
        const value =
          typeof onFulfilled === 'function' ? onFulfilled(this.#value) : this.#value
        resolve(value)
      }
      const rejectionTask = () => {
        const value =
          typeof onRejected === 'function' ? onRejected(this.#value) : this.#value
        resolve(value)
      }

      addToTaskQueue(() => {
        this.#handle({
          onFulfilled: fullfillmentTask,
          onRejected: rejectionTask,
        })
      })
    })

  return nextPromise
  }
}