While loop with promises

var Q = require("q");

// `condition` is a function that returns a boolean
// `body` is a function that returns a promise
// returns a promise for the completion of the loop
function promiseWhile(condition, body) {
    var done = Q.defer();

    function loop() {
        // When the result of calling `condition` is no longer true, we are
        // done.
        if (!condition()) return done.resolve();
        // Use `when`, in case `body` does not return a promise.
        // When it completes loop again otherwise, if it fails, reject the
        // done promise
        Q.when(body(), loop, done.reject);
    }

    // Start running the loop in the next tick so that this function is
    // completely async. It would be unexpected if `body` was called
    // synchronously the first time.
    Q.nextTick(loop);

    // The promise
    return done.promise;
}


// Usage
var index = 1;
promiseWhile(function () { return index <= 11; }, function () {
    console.log(index);
    index++;
    return Q.delay(500); // arbitrary async
}).then(function () {
    console.log("done");
}).done();

http://stackoverflow.com/questions/17217736/while-loop-with-promises