手写promise

class MyPromise {
  constructor(fn) {
    this.state = 'pending'
    this.value = null
    this.resolveCallbacks = []
    this.rejectCallbacks = []

    const resolve = (value) => {
      if (this.state === 'pending') {
        this.state = 'fulfilled'
        this.value = value
        this.resolveCallbacks.forEach(fn => fn())
      }
    }

    const reject = (value) => {
     if (this.state === 'pending') {
        this.state = 'rejected'
        this.value = value
        this.rejectCallbacks.forEach(fn => fn())
      }
    }

    try {
      fn(resolve, reject)
    } catch (e) {
      reject(e)
    }
  }
  then(onFulfilled, onRejected) {
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    }
    if (this.state === 'rejected') {
      onRejected(this.value);
    }
    if (this.state === 'pending') {
      this.resolveCallbacks.push(() => {
        onFulfilled(this.value);
      });
      this.rejectCallbacks.push(() => {
        onRejected(this.value);
      });
    }
    return this;
  }
  catch(onRejected) {
    return this.then(null, onRejected);
  }
}
const p = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve('success')
  }, 1000)
})
p.then(res => {
  console.log(res)
})
#手写promise

手写promise
