JavaScript doesn't pause for the data to come back, resulting in an undefined value

Question

JavaScript doesn't pause for the data to come back, resulting in an undefined value

When I call the function "classTableData" on a button click, my allData variable becomes undefined. The problem seems to be that it processes the next line of code without waiting for the results, causing allData to remain empty. Can anyone provide some assistance with this issue?

function classTableData(){
     loadStudentPDFData().then(function (results) {
                  studentPDFData = results.pdfData;
                });
        var allData = studentPDFData;
        $log.log("AllData"+allData.length);

    }

function loadStudentPDFData() {
  var deferred = q.defer();
   var core5PDFData=angular.copy(core5InstructionPlanner);
       core5PDFData.loadPDFSection('all', function() {
     deferred.resolve({pdfData:core5PDFData.dataSource('all').data()});
      });
    return deferred.promise;
  }

javascript angularjs promise

Answer 1

Answer №1

loadStudentPDFData function will return a promise.

Once the promise is resolved, it will return the object

{pdfData:core5PDFData.dataSource('all').data()}

.

It's important to note that placing

studentPDFData = results.pdfData;

inside the resolve callback is crucial to avoid issues that may arise due to asynchronous nature of JavaScript.

    var allData = studentPDFData;
    $log.log("AllData"+allData.length);

When these two lines are outside the callback, there's a risk of null being logged because studentPDFData might be undefined at that point.

To ensure proper execution, move these lines inside the callback as shown below:

 loadStudentPDFData().then(function (results) {
      studentPDFData = results.pdfData;
      var allData = studentPDFData;//you don't need this variable
      $log.log("AllData"+allData.length);
 });

JavaScript utilizes Promises for handling asynchronous operations effectively. Promises allow actions to be completed based on delayed results, such as those from database calls or HTTP requests.

For further information on promises, you can refer to this resource, and for details about the q library, visit this link.

Answer 2

loadStudentPDFData function will return a promise.

Once the promise is resolved, it will return the object

{pdfData:core5PDFData.dataSource('all').data()}

.

It's important to note that placing

studentPDFData = results.pdfData;

inside the resolve callback is crucial to avoid issues that may arise due to asynchronous nature of JavaScript.

    var allData = studentPDFData;
    $log.log("AllData"+allData.length);

When these two lines are outside the callback, there's a risk of null being logged because studentPDFData might be undefined at that point.

To ensure proper execution, move these lines inside the callback as shown below:

 loadStudentPDFData().then(function (results) {
      studentPDFData = results.pdfData;
      var allData = studentPDFData;//you don't need this variable
      $log.log("AllData"+allData.length);
 });

JavaScript utilizes Promises for handling asynchronous operations effectively. Promises allow actions to be completed based on delayed results, such as those from database calls or HTTP requests.

For further information on promises, you can refer to this resource, and for details about the q library, visit this link.

Answer 3

Answer №2

Execute the task inside .then()

fetchStudentFiles()
.then(function (response) {
  $log.log("Total files: " + response.fileList.length);
  return response.fileList;
});

If another .then() is attached to classListData, then return the result of fetchStudentFiles() from the classListData() function

function classListData() {
  return fetchStudentFiles()
         .then(function (response) {
           $log.log("Total files: " + response.fileList.length);
           return response.fileList;
         });
}

Answer 4

Execute the task inside .then()

fetchStudentFiles()
.then(function (response) {
  $log.log("Total files: " + response.fileList.length);
  return response.fileList;
});

If another .then() is attached to classListData, then return the result of fetchStudentFiles() from the classListData() function

function classListData() {
  return fetchStudentFiles()
         .then(function (response) {
           $log.log("Total files: " + response.fileList.length);
           return response.fileList;
         });
}

JavaScript doesn't pause for the data to come back, resulting in an undefined value

Answer №1

Answer №2

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