Can this promise be chained more cleanly?

Question

The below code uses Promises/A+ (specifically rsvp.js) to generate a token. It does so by either taking an existing token or downloading and parsing a token from a web page. Is it possible to chain this to LoadURIPromise more cleanly? Usually one can chain together promises without causing as much indentation drift.

new RSVP.Promise(function (resolve) {
    if (TokenData) {
        resolve(TokenData);
    } else {
        LoadURIPromise(TokenURL).then(function (xmlDoc) {
            TokenData = xmlDoc.evaluate(
                '/root/theToken', 
                xmlDoc.documentElement, 
                null, 
                XPathResult.FIRST_ORDERED_NODE_TYPE, 
                null
            ).singleNodeValue.textContent;
            resolve(TokenData);
        });
    }
}).then(function (token) {
    //Use Token here
}

Answer 1

I have never used RSVP, I am more familiar with Q, but I believe following should do what you need:

var evaluate = function (xmlDoc) {
    TokenData = xmlDoc.evaluate(
        '/root/theToken',
    xmlDoc.documentElement,
    null,
    XPathResult.FIRST_ORDERED_NODE_TYPE,
    null).singleNodeValue.textContent;
    return TokenData;
}

RSVP.resolve(TokenData || LoadURIPromise(TokenURL).then(evaluate))
    .then(function (token) {
    //Use Token here
    console.info(token);
});

Here is working jsFiddle

BTW, do you really need TokenData to be global?

Answer 2

I'm not familiar with the RSVP.js library. I use jQuery here, but the overall concept should still be (or exactly) the same:

// You can pull out your config to an array and use apply to use them
var config = [
  '/root/theToken',
  xmlDoc.documentElement,
  null,
  XPathResult.FIRST_ORDERED_NODE_TYPE,
  null
];

// So basically, the idea is the same, but we just pulled out some stuff
// - A ternary instead of an if statement
// - Removed the config to an array, and use apply to use them
$.Deferred(function (deferred) {
  TokenData ? deferred.resolve(TokenData) : LoadURIPromise(TokenURL).done(function(xmlDoc) {
    TokenData = xmlDoc.evaluate.apply(xmlDoc,config).singleNodeValue.textContent;
    deferred.resolve(TokenData);
  });
}).done(function (token) {
  /*We got token*/
})