javascript - 跳出 promise 链或走不同的道路

Question

我有一个问题，我可能需要因为某些值(value)而跳出整个 promise 链，或者根据一个值(value)采取两条路径。

你怎么做最好？

这是我想跳出整个链条的第一个场景。我只想给他们留言。

      DB_WorkIssues.info().then(function (details) {
     if (details.doc_count == 0 && details.update_seq == 0) {
        showMsg("This device has no local data on it and no access to the Server, please come back when you are online.")
     }  jump out here, no need to do the next set.
     else
     return;     Continue on as the values are valid.
      }).then(function () {
     return ajaxCallForJson(URI_LookupTables);
      }).then(function (json) {
     return callBulkDocLoad(DB_LookupTables, json);
      }).then(function () {
     return loadCategoriesDDL();
      }).then(function () {
     return loadEquipmentDDL();
      }).catch(function (err) {
     showMsg("Error in defineDBs: " + err);
      });

在第二种情况下，如果值是一回事，我可能想走一条路，如果值是另一回事，我可能想走另一条路。但我仍然希望链条能够与第一个 promise 一起工作。像这样:

      DB_WorkIssues.info().then(function (details) {
     if (details.doc_count == 0 && details.update_seq == 0) {

     Take this path.
          return;
          }).then(function () {
         return ajaxCallForJson(URI_LookupTables);
          }).then(function (json) {
         return callBulkDocLoad(DB_LookupTables, json);
          }).catch(function (err) {
         showMsg("Error in defineDBs: " + err);
          });

     }  
     else
     {

     Take this path instead

         return;
          }).then(function () {
         return loadCategoriesDDL();
          }).then(function () {
         return loadEquipmentDDL();
          }).catch(function (err) {
         showMsg("Error in defineDBs: " + err);
          });
    }

谢谢。

这是我在查看答案后的想法，我总是做第二个 promise ，在某些情况下只做第一个。

    DB_WorkIssues.info().then(function(details) {

      // promise variable , defined in conditional
      var promise;

    Would I set the promise to some default value, in case the following test fails

      if (details.doc_count == 0 && details.update_seq == 0) {
        // return this promise
        promise = ajaxCallForJson(URI_LookupTables).then(function(json) {
          return callBulkDocLoad(DB_LookupTables, json);
        });

      }
      return promise;

    }).then(function () {
       return loadCategoriesDDL();
    }).then(function () {
       return loadEquipmentDDL();
    }).then(function () {
       return loadLocationsDDL();
    }).catch(function (err) {
       showMsg("Error in defineDBs: " + err);
    });

我能做到吗？

谢谢。

Answer 1

我认为这是一个骨架，代表了您的目标。 Promise 非常强大，值得研究。我尝试添加有用的评论，但我建议尝试使用代码并了解正在发生的事情。

// Six named promise-creators. When called with (x), will create a promise
// which waits 200ms and then logs and resolves with (x). 
// These could represent any asynchronous operation.
const p1 = p2 = p3 = p4 = p5 = p6 = 
  (x) => {
    const p = new Promise((resolve, reject) => {
      setTimeout(() => {resolve(x); console.log(x)}, 200)
  });
  return p;
}

// A function which, when called, will execute first promise chain.
const first_steps = () => 
  p1(1)
    .then(result => p2(2))
    .then(result => p3(3))

// A function which, when called, will execute second promise chain.
const second_steps = () => 
  p4(4)
    .then(result => p5(5))
    .then(result => p6(6))

// When true, this prints numbers 1-6.
// When false, only prints numbers 4-6.
if (false) {
 console.log(first_steps().then(second_steps));
} else {
 second_steps();
}