How to cleanly extract a callback out of a when.js promise?

Question

I need to pass a callback to a function whose signature is function('ui', {foo: bar, callback: callbackfn}). The function I want to pass is a When.js promise.

The best I've come up with:

var d = when.defer();
var p = when(d);
var q = p.then(function() {
    return loadItem(newCatalogItem, name, fileOrUrl);
});

ConfirmationMessage.open('ui', { callback: d.resolve });

return q;

This works (using a deferred to prevent immediate execution, and then passing the resolve function as the callback), but seems a bit convoluted.

Is there a cleaner way?

Answer 1

I think you want to just promisify that ConfirmationMessage.open method (see also the when.js docs here and there), and then use it like a promise function, chaining then calls onto it.

For your specific example, that could be (using the portable promise constructor):

return when.promise(function(resolve) {
    ConfirmationMessage.open('ui', { callback: resolve });
}).then(function(confirmResult) {
    return loadItem(newCatalogItem, name, fileOrUrl);
});