手写一个简易的Promise

Promise/A+

手写Promise都是按照 Promise/A+来完成的。每一个步骤都应该和Promise/A+对应起来。

简易版完整代码

class MyPromise {
  //私有属性
  #status = 'pending'
  constructor(fn) {
    this.queue = []
    const resolve = (data) => {
      this.#status = 'fulfilled'
      const f1f2 = this.queue.shift()
      if (!f1f2 || !f1f2[0]) return
      const x = f1f2[0].call(undefined, data)
      if (x instanceof Promise2) {
        x.then((data) => {
          resolve(data)
        }, (reason) => {
          reject(reason)
        })
      } else {
        resolve(x)
      }
    }
    const reject = (reason) => {
      this.#status = 'rejected'
      const f1f2 = this.queue.shift()
      if (!f1f2 || !f1f2[1]) return
      const x = f1f2[1].call(undefined, reason)
      if (x instanceof Promise2) {
        x.then((data) => {
          resolve(data)
        }, (reason) => {
          reject(reason)
        })
      } else {
        resolve(x)
      }
    }
    fn.call(undefined, resolve, reject)
  }
  // 链式调用未完成。目前思路是通过状态来分类讨论。
  then(f1, f2) {
    this.queue.push([f1, f2])
  }
}
// 测试代码
const p = new MyPromise(function (resolve, reject) {
  setTimeout(function () {
    reject('出错')
  }, 3000)
})

p.then((data) => { console.log(data) }, (r) => { console.error(r) })