Return already resolved promise

Question

I have an existing project that has a lot of asynchronous functions that return promises. I\'m adding some caching so that in some cases the asynchronous functions will complet

Answer 1

Simply use return $.when(); to return an already resolved promise.

If you don't pass it any arguments at all, jQuery.when() will return a resolved promise.

Reference: https://api.jquery.com/jquery.when/

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Notes:

• This is the same as return $.when(undefined); which leads to the following rather cool trick which avoids any use of arrays and apply.

If you want to wait for a variable number of promises to complete in parallel then you can use this pattern in a loop:

var promise;   // Undefined is treated as an initial resolved promise by $.when
while (insomeloop){
    promise = $.when(promise, newpromise);
}

then make a final call on completion with:

promise.then(function(){
    // All done!
});

e.g.

var promise;$.when
var $elements = $('.some-elements');
$elements.each(function(){
     var $element = $(this).fadeout();
     promise = $.when(promise, $element.promise());
});
promise.then(function(){
    // All promises completed!
});

The downsides are minor:

• each call to when wraps the previous promise in a new promise. A minor overhead and you no longer need to maintain and evaluate an array of promises.
• no values can be directly passed through to the final function. As you typically do not want the return values from parallel promises, this is minor.
• failure of a single promise will abort the final wait early, so you need to ensure all promises will be fulfilled.