Question

I need to run an arbitrary number of scripts. The next one can run only of the previous has loaded and executed. I know RequireJS (and related) would be the correct choice, but I’m trying to learn about promises, so this is my experiment:

var files = [
  'first.js',
  'second.js',
  'third.js',
  'fourth.js'
];

var funcs = files.map(function(file) {
  return function() { return $.getScript(file); }
});

var deferred = $.Deferred();

funcs.reduce(function (soFar, f) {
   return soFar.then(f);
}, deferred.resolve(funcs[0])); 

Can someone elaborate about the pitfalls and alternatives to my solution?

Answer 1

What you’re really looking for is .pipe ( or in 1.8+, I believe .then was changed to mean the same thing )

In short, pipe will allow you to chain promises in the way that you’re looking for. The code might look something like this ( untested ):

var files, scriptsLoaded;

files = [ 'first.js', 'second.js', 'third.js', 'fourth.js' ];

while( files.length ) {
    (function() {
        var currentUrl = files.shift();

        scriptsLoaded = scriptsLoaded ?
            scriptsLoaded.pipe(function() {
                return $.getScript( currentUrl );
            }) :
            $.getScript( currentUrl );
    }());
}

$.when( scriptsLoaded ).done(function() {
    // All scripts are now loaded assuming none of them failed
});

** Edit **

With that link you provided, I understand what you were trying to accomplish. Here’s a corrected version of your solution with some comments. It accomplishes the same thing as the other solution, however it’s a much more concise version:

var files = [ 'first.js', 'second.js', 'third.js', 'fourth.js' ];

// The initial value provided to the reduce function is a promise
// that will resolve when the first file has been loaded.  For each
// of the remaining file names in the array, pipe it through that first
// promise so that the files are loaded in sequence ( chained ).
// The value that is returned from the reduce function is a promise
// that will resolve only when the entire chain is done loading.
var scriptsLoaded = files.slice(1).reduce(function (soFar, file) {
    return soFar.pipe(function() {
        return $.getScript( file );
    });
}, $.getScript( files[0] );