A simple promise implementation

a-simple-promise.js

// pending
// fulfilled
// rejected

class MyPromise {
  constructor(executor) {
    this.$state = "pending"
    this.$res = null
    this.$chain = []

    const resolve = res => {
      if(res && res.then && typeof res.then === "function") {
        res.then(resolve, reject)
        return
      }

      if(this.$state !== "pending") return
      this.$state = "fulfilled"

      this.$res = res

      for(let {onFulfilled} of this.$chain) {
        onFulfilled && onFulfilled(res)
      }
    }

    const reject = res => {
      if(this.$state !== "pending") return
      this.$state = "rejected"
      this.$res = res

      for(let {onRejected} of this.$chain) {
        onRejected && onRejected(res)
      }
    }

    executor(resolve, reject)
  }

  then1(onFulfilled, onRejected) {
    if(this.$state === "fulfilled") {
      setImmediate(onFulfilled)
    } else if(this.$state === "rejected") {
      setImmediate(onRejected)
    } else {
      this.$chain.push({onFulfilled, onRejected})
    }
  }

  then(onFulfilled, onRejected) {
    return new MyPromise((resolve, reject) => {
      const _onFulfilled = res => {
        resolve(onFulfilled(res))
      }

      const _onRejected = res => {
        reject(onRejected(res))
      }

      if(this.$state === "fulfilled") {
        setImmediate(() => {
          _onFulfilled(this.$res)
        })
      } else if(this.$state === "rejected") {
        setImmediate(() => {
          _onRejected(this.$res)
        })
      } else {
        this.$chain.push({onFulfilled: _onFulfilled, onRejected: _onRejected})
      }
    })
  }
}

function test1() {
  new MyPromise((resolve, reject) => {
    setTimeout(function() {
      resolve(1)
    }, 10)
  }).then(console.log)

  new MyPromise((resolve, reject) => {
    setTimeout(function() {
      reject(2)
    }, 10)
  }).then(null, console.log)
}

function test2() {
  new MyPromise((resolve, reject) => {
    resolve(1)
  })
    .then(n => {
      console.log(n)
      return n + 100
    })
    .then(n => {
      console.log(n)
      return n + 200
    })
    .then(n => {
      console.log(n)
      return n + 300
    })
}

function test3() {
  const m = n => new MyPromise((resolve, reject) => {
    setTimeout(() => {
      console.log("finish m", n)
      resolve(n)
    }, 10)
  })

  m(1)
    .then(n => {
      console.log(n)
      return m(2)
    })
    .then(n => {
      console.log(n)
    })
}

// test1()
// test2()
// test3()