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ES6 Promises in NodeJS

· 2 min read

NodeJS SDKs will now return promises in endpoint calls, as an alternative to accepting callbacks.

  • We use ES6 Promises which is based on Promises/A+ and is interoperable with most Promise-based implementations.
  • Callbacks are still supported (for backward-compatibility) but are now optional.

Re-generate your NodeJS SDK to get the new Promise-enabled SDK implementation now.

Why Promises?​

Here is an except from a blog by StrongLoop:

Callbacks are the simplest possible mechanism for asynchronous code in JavaScript. Unfortunately, raw callbacks sacrifice the control flow, exception handling, and function semantics familiar from synchronous code. Promises provide a way to get those things back.

Usage​

All endpoint methods now return Promises.

let promise = apiController.endpointMethod();
promise.then(onFulfilled, onRejected);
  • onFulfilled method is called if the endpoint call succeeds and onRejected is called upon error.
  • onFulfilled method will be passed a response from the endpointMethod if any, after validation.
  • onRejected method will be passed an error object containing an errorMessage and errorCode or a custom error object if defined by your API description.

The sample code above can also be rewritten as:

apiController.endpointMethod()
    .then(onFulfilled)
    .catch(onRejected);
Order of callback execution

An endpoint call's callback is always executed before its Promise is resolved.

Advanced Usage​