JavaScript async返回'then not defined‘

Question

我是异步编程的新手，我读过类似的线程，它们似乎都有相同的问题，不返回任何东西，而我的……？

但是，我仍然收到错误消息“Cannot read property”then“of undefined”

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();

        });
}

上面输出的调试消息很好，它从服务器获取数据，当我在其他地方调用它时，它会出错

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，所以它隐式返回未定义的。此外，由于queryClient.queryByWiql(query, projectId)已经返回了promise，因此您不需要创建自己的promise，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);
}

如果您想保留日志语句和notifyLoadSucceeded，您可以这样做，但您仍然不需要创建自己promise。如果你在.then回调中，你只需要返回你希望promise解析的值：

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;
    });
}