怎么将一个函数内部的事件监听函数返回的值返回出来

Question

1.代码如下，我希望从seclect中返回请求成功后取得的结果，请求是异步的，有什么办法或者替代办法？

2.

function select(key,store) {   // 怎么保证这个异步结果的返回...
    var request3 = store.get(key);

      request3.onerror = function(event) {
          console.log("Error : " + event.errorCode);
      };
      request3.onsuccess = function(event) {
          return event.target.result;
      };
}

Answer 1

1.回调函数

function select(key,store, callback) {
    var request3 = store.get(key);

      request3.onerror = function(event) {
          console.log("Error : " + event.errorCode);
      };
      request3.onsuccess = function(event) {
          callback(event.target.result);
      };
}

// 调用
select(key,store, function(result) {
    console.log(result)
})

2.Promise

function select(key,store, onsuccess) {
    var request3 = store.get(key);
    
    return new Promise(function(resolve, reject) {
          request3.onerror = function(event) {
              reject(event);
          };

          request3.onsuccess = function(event) {
              resolve(event.target.result);
          };

   })
}

// 调用
select(key,store).then(function(result){
    // success
    console.log(result)
}, function(e){
    // error
    console.log(e)
})

3.Event

// 监听自定义事件
window.addEventListener('selectSuccess', function(e){
   console.log(e.result)
})

function select(key,store, onsuccess) {
    var request3 = store.get(key);

      request3.onerror = function(event) {
          console.log("Error : " + event.errorCode);
      };
      request3.onsuccess = function(event) {
          // 触发事件
          var myEvent = new Event('selectSuccess');
          myEvent.result = event.target.result;
          window.dispatchEvent(myEvent);
      };
}

Answer 2

有jquery的话，用$.Deferred方法

function select(key,store) {   // 怎么保证这个异步结果的返回...
    var request3 = store.get(key);
    var deferred = $.Deferred();

    request3.onerror = function(event) {
        console.log("Error : " + event.errorCode);
        deferred.reject("Error : " + event.errorCode);
    };

    request3.onsuccess = function(event) {
      deferred.resolve(event.target.result);
    };

    return deferred.promise();
  }

  select(key, store).then(function(result){
    // result
  });