Threads can be added to and removed from a napi_threadsafe_function object during its existence. Thus, in addition to specifying an initial number of threads upon creation, napi_acquire_threadsafe_function can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, napi_release_threadsafe_function can be called to indicate that an existing thread will stop making use of the thread-safe function.

    napi_threadsafe_function objects are destroyed when every thread which uses the object has called napi_release_threadsafe_function() or has received a return status of napi_closing in response to a call to napi_call_threadsafe_function. The queue is emptied before the napi_threadsafe_function is destroyed. napi_release_threadsafe_function() should be the last API call made in conjunction with a given napi_threadsafe_function, because after the call completes, there is no guarantee that the napi_threadsafe_function is still allocated. For the same reason, do not use a thread-safe function after receiving a return value of napi_closing in response to a call to napi_call_threadsafe_function. Data associated with the napi_threadsafe_function can be freed in its napi_finalize callback which was passed to napi_create_threadsafe_function(). The parameter initial_thread_count of napi_create_threadsafe_function marks the initial number of aquisitions of the thread-safe functions, instead of calling napi_acquire_threadsafe_function multiple times at creation.

    Once the number of threads making use of a napi_threadsafe_function reaches zero, no further threads can start making use of it by calling napi_acquire_threadsafe_function(). In fact, all subsequent API calls associated with it, except napi_release_threadsafe_function(), will return an error value of napi_closing.

    The thread-safe function can be “aborted” by giving a value of napi_tsfn_abort to napi_release_threadsafe_function(). This will cause all subsequent APIs associated with the thread-safe function except napi_release_threadsafe_function() to return napi_closing even before its reference count reaches zero. In particular, napi_call_threadsafe_function() will return napi_closing, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of napi_closing from napi_call_threadsafe_function() a thread must not use the thread-safe function anymore because it is no longer guaranteed to be allocated.