readability.md

var items = files.getLastTwoVersions(filename);
var versions = .then(function(items) {
    return [versions.get(items.last), 
            versions.get(items.previous)];
});
var diff = versions.spread(function(v1, v2) { 
    return diffService.compare(v1.blob, v2.blob)
});
var result = diff.then(function(diff) {
    // voila, diff is ready. Do something with it.
});

vs

let items = files.getLastTwoVersions(filename)

let versions = items.then(
    items => [versions.get(items.last), versions.get(items.previous)])

let diff = versions.spread(
    (v1, v2) => diffService.compare(v1.blob, v2.blob))

let result = diff.then(
    diff => /* voila, diff is ready. Do something with it. */ );