自己实现简易版Promise

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function Promise(fn){
    var status = 'pending'
    function successNotify(){
        status = 'resolved'
        toDoThen.apply(undefined, arguments)
    }
    function failNotify(){
        status = 'rejected'
        toDoThen.apply(undefined, arguments)
    }
    function toDoThen(){
        setTimeout(()=>{ // 保证回调是异步执行的
            if(status === 'resolved'){
                for(let i =0; i< successArray.length;i ++)    {
                    successArray[i].apply(undefined, arguments)
                }
            }else if(status === 'rejected'){
                for(let i =0; i< failArray.length;i ++)    {
                    failArray[i].apply(undefined, arguments)
                }
            }
        })

    }
    var successArray = []
    var failArray = []
    fn.call(undefined, successNotify, failNotify)
    return {
        then: function(successFn, failFn){
            successArray.push(successFn)
            failArray.push(failFn)
            return undefined // 简化
        }
    }
}