In HTML5 is there any way to make Filereader's readAsBinaryString() synchronous - html

Question

How to wait till onload event completes
function(){
var filedata=null;
reader.onload=function(e){
filedata=e.target.result;
};
reader.readAsBinaryString(file);
//I need code to wait till onload event handler gets completed.
return filedata;
}

Answer 1

Typical solution to this is to separate your code so, that the part which uses the loaded data is in a separate function, which is then called from the event.
Pseudo-code'ish:
function load() {
//load here
reader.onload = function(e) {
process(e.target.result);
}
}
function process(result) {
//finish working here
}

Answer 2

You can read synchronously using threads (Webworkers in Javascript).
http://www.w3.org/TR/FileAPI/#readingOnThreads

Answer 3

<script type="text/javascript" src="http://code.jquery.com/jquery-1.8.0.min.js"></script>
<form><input type="file" id="files" name="file" onchange="upload()" /></form>
function readFile(dfd) {
bytes = [];
var files = document.getElementById('files').files;
if (!files.length) {
alert('Please select a file!');
return;
}
var file = files[0];
var reader = new FileReader();
// If we use onloadend, we need to check the readyState.
reader.onloadend = function(evt) {
if (evt.target.readyState == FileReader.DONE) { // DONE == 2
var content = evt.target.result;
//bytes = stringToBytes(content);
dfd.resolve(content);
}
};
reader.readAsBinaryString(file);
}
function upload() {
var dfd = new $.Deferred();
readFile(dfd);
dfd.done(function(content){
alert("content:" + content);
});
}

Answer 4

The answer by Jani is correct, but in the case of dropping multiple files at once in a droparea, there are not separate events for each file (as far as I know). But the idea may be used to load the files synchronously by recursion.
In the code below I load at number of files and concatenate their content into a single string for further proccesing.
var txt="", files=[];
function FileSelectHandler(e) {
e.preventDefault();
files = e.target.files || e.dataTransfer.files;
txt="";
readFile(0);
}
function readFile(n) {
file=files[n];
var reader = new FileReader();
reader.onload = function() {
txt += reader.result;
}
reader.readAsText(file);
if (n<files.length-1) { readFile(n+1); }
else { setTimeout(doWhatEver, 100);}
}
function doWhatEver(){
outtext.innerHTML=txt;
}
The last file also needs a bit of extra time to load. Hence the "setTimeout".
"outtext" is the handle to a textarea where the entire string is displayed. When outputting in at textarea the browser wont' parse the string. This makes it possible to view not only text but also html, xml etc.

Answer 5

No there is not. All IO operations must be asynchronous in Javascript.
Making file operation synchronous would effectively block the browser UI thread freezing the browser. The users don't like that.
Instead you need to design your script in asynchronous manner. It is quite easy with Javascript.