Promise手寫

function Promise(executor){

    //在內部中定義一個狀態,當前promise狀態
    let self = this
    self.value = undefined
    self.reason = undefined
    self.status = 'pending' //默認promise狀態是padding

    function resolve(value){
        if(self.status === 'pending'){
            self.value = value
            self.status = 'resolved';       //成功了
        }
    }

    function reject(reason){
        if(self.status === 'pending'){
            self.reason = reason
            self.status = 'rejected'    //失敗態
        }       
    }

    executor(resolve,reject)
}

Promise.prototype.then = function (onFulfilled,onRejected){
    let self = this
    if(self.status === 'resolved') {
        onFulfilled(self.value)
    }

    if(self.status === 'rejected') {
        onRejected(self.reason)
    }

}

module.exports = Promise
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