In JavaScript how to force a function finish execution before return some data

Question

I want to use PhoneGap to read a file then return me the data of that file, but when i execute the code, it return before read the file. The code below shows how to read file:

function getData() {
    var fileData = null;
    window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, gotFS, fail);
      function gotFS(fileSystem) {
         fileSystem.root.getFile("data.txt", null, gotFileEntry, fail);
         function gotFileEntry(fileEntry) {
            fileEntry.file(gotFile, fail);
         }
         function gotFile(file){
            readAsText(file);
         }

        function readAsText(file) {
           var reader = new FileReader();
           reader.onloadend = function(evt) {
            fileData = evt.target.result;
                alert(fileData) 
               // this alert is executed after returning the data, so it return the data, but the second alert return null when this is method called.
           };
           reader.readAsText(file);
        }
    }
    return fileData;
}

Now when i call that function, it will return null

var data = getData();
alert(data); // return null

score 1 · Accepted Answer · edited Dec 08 '13 at 11:01

1

This is an asynchronous method so you'll have to use a callback.

Change getData so it accepts a callback:

function getData(callback) { ....

Then where you are alerting, do this instead:

if (callback) callback(fileData);

Now you can call your function like so:

var _data;

getData(function(data) { 
 // do stuff with data ...
 _data = data;
 alert(data);
});

edited Dec 08 '13 at 11:01

nnnnnn

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answered Dec 08 '13 at 10:23

HaukurHaf

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Thanks for the edit, looks much better now. I wrote the original answer on my mobile phone so it was a bit quick'n'dirty :-) – HaukurHaf Dec 08 '13 at 11:38

score 0 · Answer 2 · edited May 23 '17 at 12:23

0

You can't have synchonously the result of an asynchonous query.

check the first response of that for more details : How do I return the response from an asynchronous call?

edited May 23 '17 at 12:23

Community

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answered Dec 08 '13 at 10:33

Shivaan Keldon

51
6

In JavaScript how to force a function finish execution before return some data

2 Answers2