Passing more than single value through promise chain

Promises are fulfilled (resolved or rejected) with a single value. If we need to keep track of multiple values we can return several values as an array, even using .spread method for convenience

promise
    .then(function (x) {
        // compute y
        return [x, y];
    })
    .spread(function (x, y) {
        // keep accumulating values
    });

Passing multiple values using separate arguments quickly becomes cumbersome. Some promise implementation, for example AngularJS $q do not implement .spread method, forcing to use an array argument

$qPromise
    .then(function (x) {
        return [x, y];
    })
    .then(function (results) {
        // results[0] => x
        // results[1] => y
    });

We can use a helper method that takes a function expecting separate arguments and create new function that splits an array.

function spread(fn) {
    return function (arr) {
        return fn.apply(null, arr);
    };
}
$qPromise
    .then(function (x) {
        return [x, y];
    })
    .then(spread(function (x, y) {
        // convenient
    }));

Even when .spread method is available, passing all values through every function is inconvenient. We might generate a value Z at the step N and use it only at the step N + K. It would be nice to avoid passing the value Z through functions at steps N + 1 through N + K - 1.

I use a separate object for this. Since the promise chain is usually constructed inside a function, the separate object is a local variable scoped to the function.

function action() {
    var data = {};
    return promise
        .then(function () {
            var x = // generate x
            var y = // generate y
            // we might need x and y somewhere down the road
            data.x = x;
            data.y = y;
            // the next step in the chain only needs the x + y sum
            return x + y;
        })
        .then(function (sum) {
            console.log('sum =', sum);
        })
        .then(function printXandY() {
            // now use x and y
            console.log('x =', data.x, 'y =', data.y);
        });

If we want, we could return the full object in the last step of the promise chain.