JavaScript 异步返回 'then not defined'

Question

我是异步编程的新手，读过类似的线程，这些线程似乎都具有不返回任何内容的相同问题，而我的却......？

但是我仍然收到错误消息“无法读取未定义的属性‘then’”

function getWorkItems(iterationPath, projectId) {

    var queryClient = VSS_Service.getCollectionClient(TFS_Wit_QueryAPI.WorkItemTrackingHttpClient);
    var query = { query: "SELECT [System.Id] FROM WorkItem WHERE [System.IterationPath] = '" + iterationPath + "'" };
    var resultOfQuery;

    queryClient.queryByWiql(query, projectId).then(

        function (resultOfQuery) {
            return new Promise((resolve, reject) => {
                resolve(resultOfQuery);
                console.log("Debug: " + JSON.stringify(resultOfQuery));
            })
            VSS.notifyLoadSucceeded();

        });
}

上面打印了调试消息，它从服务器获取数据，然后当我在其他地方调用它时出现错误

let bar;

getWorkItems(counter.path, projectId).then ( res => {
    bar = res;
    console.log("Debug: should be output of query " + JSON.stringify(bar));
})

无法读取未定义的属性“then”

Answer 1

getWorkItems 中没有 return 语句，因此它隐式返回 undefined。此外，由于 queryClient.queryByWiql(query, projectId) 已经返回 Promise，因此您无需创建自己的 Promise，并且可以执行以下操作:

function getWorkItems(iterationPath, projectId)
{
  var queryClient = VSS_Service.getCollectionClient(TFS_Wit_QueryAPI.WorkItemTrackingHttpClient);
  var query = { query: "SELECT [System.Id] FROM WorkItem WHERE [System.IterationPath] = '" + iterationPath + "'" };
  return queryClient.queryByWiql(query, projectId);
}

如果您想保留该日志语句并notifyLoadSucceeded 在那里，您可以，但您仍然不需要创建自己的 promise 。如果您处于 .then 回调中，则只需返回您希望 Promise 解析为的值:

function getWorkItems(iterationPath, projectId)
{
  var queryClient = VSS_Service.getCollectionClient(TFS_Wit_QueryAPI.WorkItemTrackingHttpClient);
  var query = { query: "SELECT [System.Id] FROM WorkItem WHERE [System.IterationPath] = '" + iterationPath + "'" };
  return queryClient.queryByWiql(query, projectId)
    .then(function(resultOfQuery) {
      console.log("Debug: " + JSON.stringify(resultOfQuery));
      VSS.notifyLoadSucceeded();
      return resultOfQuery;
    });
}