7

In my script, I need to retrieve a dictionary to convert encoded values into names:

$.ajax({
    // retrieve dictionary
})
.done(function(dictionary){
    // convert encoded values into names
})
.done(function(){
    // run my application
});

However, sometimes the dictionary has already been loaded by another application, and in this case I don't need the ajax call:

if (dictionary) {
    // convert encoded values into names
    // run my application
}
else {
$.ajax({
    // retrieve dictionary
})
.done(function(dictionary){
    // convert encoded values into names
})
.done(function(){
    // run my application
});
}

This if/else statement is rather heavy, is there a way to make it shorter:

// load dictionary if needed
// then run my application

Note: I used the $ sign for my pseudo-code, but I am not necessarily tied to jQuery.

Christophe
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    I do this sort of thing quite a lot in my applications. The thing you have you get your mind round is that if data is acquired asynchronously then you MUST access it with asynchronous methods regardless of whether it is already acquired or not - reason being at each and every usage, you don't know whether the data is immediately available or whether it needs to be fetched. – Beetroot-Beetroot Sep 30 '13 at 22:05
  • @Beetroot-Beetroot could you explain how you access already acquired data with asynchronous methods? Maybe post an answer with code or pseudo-code? – Christophe Sep 30 '13 at 22:14
  • See the answers already posted. That's what the guys are trying to explain. – Beetroot-Beetroot Sep 30 '13 at 22:15

3 Answers3

3

You should call $.ajax() exactly once, and store the returned promise in your (global-ish) dictionary variable.

Then, every time you want to use the result, just write dictionary.then(...).
If the AJAX request already finished, the callback will run immediately.

SLaks
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    Well, that's not how it works. As I said in the question, even when my code starts the dictionary might already be present (loaded by another script not under my control), and in this case I don't even need the first ajax call. For example the dictionary could be stored in localStorage. – Christophe Sep 30 '13 at 16:51
  • @Christophe: Then set the global to a pre-resolved promise and call `.then()` anyway. – SLaks Sep 30 '13 at 16:52
  • ok, that's what I am trying to understand, how to write a conditional promise. – Christophe Sep 30 '13 at 16:53
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    @Christophe: `var dictionary = $.when(some object);` – SLaks Sep 30 '13 at 16:54
3

This is the pattern I generally use:

var done = function() {
    //continue loading application
}

if(!dictionary) {
    $.ajax({

    })
        .done(done);
} else {
    done.apply(this);
}

A very similar pattern, that always makes use of a deferred object, could be the following:

var 
    dictionaryDeferred = new $.Deferred(),
    dictionaryPromise = dictionaryDeferred.promise();

if(!dictionary) {
    $.ajax({

    })
        .done(function() {
            //do something with the response
            dictionaryDeferred.resolve();
        });
} else {
    dictionaryDeferred.resolve();
}

dictionaryPromise.then(function() {
    //continue loading application
});
Adam Jenkins
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  • Thanks. I like the second pattern better, as it makes chaining easier. One issue might be that the resolved value of dictionaryPromise is not consistent between the two branches (but not a problem in my case as dictionary is a global variable). – Christophe Sep 30 '13 at 17:14
  • You don't have to use `dictionaryPromise` in both places where dictionary might be requested, you can just use it in this place where the application is being loaded, if you wish, because you are still checking for `if(!dictionary)` already exists here or not. – Adam Jenkins Sep 30 '13 at 17:25
  • Why to use 'new' with $.Deferred() when the use of 'new' is optional? – Pulak Kanti Bhattacharyya Aug 02 '14 at 08:22
  • @PulakKantiBhattacharyya - because it's optional, meaning you can use it or not. I chose to because I feel that it illustrates to all who come after me very clearly that I am getting a reference to a new instance of a `Deferred` object. This is my opinion and personal preference. Yours may be different, and that's ok. – Adam Jenkins Aug 02 '14 at 10:35
1

Maybe create a bogus promise with $.when?

var promise;
if (dictionary) promise = $.when(dictionary);
else {
    promise = $.ajax({

    })
    .done(function(dictionary){
        // convert encoded values into names
    });
}

promise
    .done(function(){
        // run my application
    });
Adam Zielinski
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