promises

N-API provides facilities for creating Promise objects as described in [Section 25.4][] of the ECMA specification. It implements promises as a pair of objects. When a promise is created by napi_create_promise(), a “deferred” object is created and returned alongside the Promise. The deferred object is bound to the created Promise and is the only means to resolve or reject the Promise using napi_resolve_deferred() or napi_reject_deferred(). The deferred object that is created by napi_create_promise() is freed by napi_resolve_deferred() or napi_reject_deferred(). The Promise object may be returned to JavaScript where it can be used in the usual fashion.

For example, to create a promise and pass it to an asynchronous worker:

napi_deferred deferred;
napi_value promise;
napi_status status;
// Create the promise.
status = napi_create_promise(env, &deferred, &promise);
if (status != napi_ok) return NULL;
// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred);
// Return the promise to JS
return promise;

The above function do_something_asynchronous() would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred:

napi_deferred deferred;
napi_value undefined;
napi_status status;
// Create a value with which to conclude the deferred.
status = napi_get_undefined(env, &undefined);
if (status != napi_ok) return NULL;
// Resolve or reject the promise associated with the deferred depending on
// whether the asynchronous action succeeded.
if (asynchronous_action_succeeded) {
  status = napi_resolve_deferred(env, deferred, undefined);
} else {
  status = napi_reject_deferred(env, deferred, undefined);
}
if (status != napi_ok) return NULL;
// At this point the deferred has been freed, so we should assign NULL to it.
deferred = NULL;