function getJSON(Url){

  return new Promise((resolve,reject)=>{

        request= new XMLHttpRequest();

        request.open('Get',Url);

        request.onload=function(){

            try{

               if(this.status===200){

                   resolve(JSON.parse(this.response));

               }else{

                   reject(this.status+"" +this.statusText);

               }

            }catch(e){

               reject(e.message);

            }

        }
　　　　request.onerror=function(){
　　　　　　reject(this.status+""+this.statusText);
　　　　}
　　　　request.send();
  })

}

getJSON("data/ninjas.Json").then(ninjas=>{ninjas !== null,'ninjas obtained!'}).catch(e=>console.log('there must smth make an reject'));

function async(generator){

  var iterator =generator();

  function handle(iteratorResult){

     if(iteratorResult.done){return;}

     const iteratorValue=iteratorResult.value;

     if(iteratorValue instanceof Promise){

        iteratorValue.then(res=>handle(iterator.next(res))

                          .catch(err=>iterator.throw(err));

     }

  }

  try{

     handle(iterator.next());

  }catch(e){

     iterator.throw(e);

  }

}

async(function*(){

    try{

       const ninjas= yield getJSON('data/ninjas.json');

       const missions= yield getJSON(ninjas[0].missionUrl);

       const missionDescription = yield getJSON(missions[0].detailsUrl);

       //study the missionDetails

    }catch(e){

       //we weren't able to get the mission details

    }

});

function getJSON(url,callback,err){

  request=new XMLHttpRequest();

  request.open('GET',url);

  request.onload=function(){

    try{

       if(this.status===200){

          callback(this.reponse);

       }else{

           err(this.status,this.statusText);

       }

    }

   request.onerror=function(){

      err(this.status,this.statusText);

   }

   request.send();

  }

}