Learning Node.js for .NET Developers

Writing cleaner asynchronous code using promises

Promises are an alternative pattern to callbacks for writing asynchronous code. A promise represents an operation that hasn't completed yet but is expected to do so in the future. As the name promise suggests, a promise is a contract to eventually provide a value or a reason for failure (that is, an error). You may already be familiar with this pattern from Tasks in .NET or Futures in Java. A promise has three possible states:

• pending represents an in-progress operation
• fulfilled representing a successful operation, with a result value
• rejected representing an unsuccessful operation, with a failure reason

When executing a single operation, the callback-based and promise-based approaches appear quite ...