How to force Sequential Javascript Execution?
Well, setTimeout, per its definition, will not hold up the thread. This is desirable, because if it did, it'd freeze the entire UI for the time it was waiting. if you really need to use setTimeout, then you should be using callback functions:
function myfunction() { longfunctionfirst(shortfunctionsecond);}function longfunctionfirst(callback) { setTimeout(function() { alert('first function finished'); if(typeof callback == 'function') callback(); }, 3000);};function shortfunctionsecond() { setTimeout('alert("second function finished");', 200);};If you are not using setTimeout, but are just having functions that execute for very long, and were using setTimeout to simulate that, then your functions would actually be synchronous, and you would not have this problem at all. It should be noted, though, that AJAX requests are asynchronous, and will, just as setTimeout, not hold up the UI thread until it has finished. With AJAX, as with setTimeout, you'll have to work with callbacks.
I am back to this questions after all this time because it took me that long to find what I think is a clean solution :The only way to force a javascript sequential execution that I know of is to use promises.There are exhaustive explications of promises at : Promises/A and Promises/A+
The only library implementing promises I know is jquery so here is how I would solve the question using jquery promises :
<html><head> <script src="http://code.jquery.com/jquery-1.9.1.min.js"></script> <script type="text/javascript"> function myfunction() { promise = longfunctionfirst().then(shortfunctionsecond); } function longfunctionfirst() { d = new $.Deferred(); setTimeout('alert("first function finished");d.resolve()',3000); return d.promise() } function shortfunctionsecond() { d = new $.Deferred(); setTimeout('alert("second function finished");d.resolve()',200); return d.promise() } </script></head><body> <a href="#" onclick="javascript:myfunction();return false;">Call my function</a></body></html>By implementing a promise and chaining the functions with .then() you ensure that the second function will be executed only after the first one has executedIt is the command d.resolve() in longfunctionfirst() that give the signal to start the next function.
Technically the shortfunctionsecond() does not need to create a deferred and return a promise, but I fell in love with promises and tend to implement everything with promises, sorry.
I am an old hand at programming and came back recently to my old passion and am struggling to fit in this Object oriented, event driven bright new world and while i see the advantages of the non sequential behavior of Javascript there are time where it really get in the way of simplicity and reusability.A simple example I have worked on was to take a photo (Mobile phone programmed in javascript, HTML, phonegap, ...), resize it and upload it on a web site.The ideal sequence is :
- Take a photo
- Load the photo in an img element
- Resize the picture (Using Pixastic)
- Upload it to a web site
- Inform the user on success failure
All this would be a very simple sequential program if we would have each step returning control to the next one when it is finished, but in reality :
- Take a photo is async, so the program attempt to load it in the img element before it exist
- Load the photo is async so the resize picture start before the img is fully loaded
- Resize is async so Upload to the web site start before the Picture is completely resized
- Upload to the web site is asyn so the program continue before the photo is completely uploaded.
And btw 4 of the 5 steps involve callback functions.
My solution thus is to nest each step in the previous one and use .onload and other similar stratagems, It look something like this :
takeAPhoto(takeaphotocallback(photo) { photo.onload = function () { resizePhoto(photo, resizePhotoCallback(photo) { uploadPhoto(photo, uploadPhotoCallback(status) { informUserOnOutcome(); }); }); }; loadPhoto(photo);});(I hope I did not make too many mistakes bringing the code to it's essential the real thing is just too distracting)
This is I believe a perfect example where async is no good and sync is good, because contrary to Ui event handling we must have each step finish before the next is executed, but the code is a Russian doll construction, it is confusing and unreadable, the code reusability is difficult to achieve because of all the nesting it is simply difficult to bring to the inner function all the parameters needed without passing them to each container in turn or using evil global variables, and I would have loved that the result of all this code would give me a return code, but the first container will be finished well before the return code will be available.
Now to go back to Tom initial question, what would be the smart, easy to read, easy to reuse solution to what would have been a very simple program 15 years ago using let say C and a dumb electronic board ?
The requirement is in fact so simple that I have the impression that I must be missing a fundamental understanding of Javsascript and modern programming, Surely technology is meant to fuel productivity right ?.
Thanks for your patience
Raymond the Dinosaur ;-)