1.promise 的简单实现

class Promise {
  constructor(executor) {
    this.status = "pending"
    this.value = undefined
    this.reason = undefined
    this.onResolvedCallbacks = []
    this.onRejectedCallbacks = []
    let resolve = (value) => {
      if (this.status === "pending") {
        this.status = "resolved"
        this.value = value
        this.onResolvedCallbacks.forEach((fn) => fn())
      }
    }
    let reject = (reason) => {
      if (this.status === "pending") {
        this.status = "rejected"
        this.reason = reason
        this.onRejectedCallbacks.forEach((fn) => fn())
      }
    }
    try {
      executor(resolve, reject)
    } catch (err) {
      reject(err)
    }
  }
  then(onFullFilled, onRejected) {
    if (this.status == "resolved") {
      onFullFilled(this.value)
    }
    if (this.status === "rejected") {
      onRejected(this.reason)
    }
    if (this.status === "pending") {
      this.onResolvedCallbacks.push(() => {
        onFullFilled(this.value)
      })
      this.onRejectedCallbacks.push(() => {
        onRejected(this.reason)
      })
    }
  }
}
const p = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("hello world")
  }, 100)
})
p.then(
  (data) => {
    console.log(data)
  },
  (error) => {
    console.log(error)
  }
)