手写Promise

/**
 * author：lsr
 * desc：手写Promise
 * date：20220118
 */
class MyPromise {
  state = 'pending'
  value = undefined
  reason = undefined
  resolveCallbacks = []
  rejectCallbacks = []
  constructor(fn) {
    const resolveHandler = (value) => {
      if (this.state === 'pending') {
        this.state = 'fulfilled'
        this.value = value
        this.resolveCallbacks.forEach(f => f())
      }
    }
    const rejectHandler = (reason) => {
      if (this.state === 'pending') {
        this.state = 'rejected'
        this.reason = reason
        this.rejectCallbacks.forEach(f => f())
      }
    }
    try {
      fn(resolveHandler, rejectHandler)
    } catch (err) {
      rejectHandler(err)
    }
  }
  then(fn1, fn2) {
    fn1 = typeof fn1 === 'function' ? fn1 : (value) => value
    fn2 = typeof fn2 === 'function' ? fn2 : (err) => err
    if (this.state === 'pending') {
      return new MyPromise((resolve, reject) => {
        this.resolveCallbacks.push(() => {
          try {
            const newValue = fn1(this.value)
            resolve(newValue)
          } catch (err) {
            reject(err)
          }
        })
        this.rejectCallbacks.push(() => {
          try {
            const newReason = fn2(this.reason)
            reject(newReason)
          } catch (err) {
            reject(err)
          }
        })
      })
    } else if (this.state === 'fulfilled') {
      return new MyPromise((resolve, reject) => {
        try {
          const newValue = fn1(this.value)
          resolve(newValue)
        } catch (err) {
          reject(err)
        }
      })
    } else if (this.state === 'rejected') {
      return new MyPromise((resolve, reject) => {
        try {
          const newReason = fn2(this.reason)
          reject(newReason)
        } catch (err) {
          reject(err)
        }
      })
    }
  }
  catch(fn) {
    return this.then(null, fn)
  }
}
MyPromise.all = function(list = []) {
  return new MyPromise((resolve, reject) => {
    let count = 0
    let results = []
    list.forEach(p => {
      p.then(res => {
        count++
        results.push(res)
        if (count === list.length) {
          resolve(results)
        }
      }).catch(err => reject(err))
    })
  })
}
MyPromise.race = function (list = []) {
  return new MyPromise((resolve, reject) => {
    let flag = false
    list.forEach(p => {
      p.then(res => {
        if (!flag) {
          flag = true
          resolve(res)
        }
      }).catch(err => reject(err))
    })
  })
}
MyPromise.resolve = function(value) {
  return new MyPromise((resolve, reject) => resolve(value))
}
MyPromise.reject = function(error) {
  return new MyPromise((resolve, reject) => reject(error))
}

标签：resolve,err,value,MyPromise,Promise,reject,catch,手写
来源： https://blog.csdn.net/qq_25436827/article/details/122566739