The document discusses callbacks and promises in JavaScript. It notes that callbacks don't have return or throw and can lead to nested code without guarantees of flow or parallelism. Promises are presented as an alternative - objects that represent an eventual fulfillment or rejection. Popular promise libraries like Q, When, and Bluebird are listed. Common JavaScript patterns like single and multiple ajax calls are shown using callbacks and promises. Promises allow values to be returned synchronously and promises to be reused. Benefits include avoiding callback hell and enabling better code reuse.