Synchronous Operation - It occurs when a process waits for another process to complete or depends on the result of the ongoing process, therefore it waits for that process to complete before the waiting process can begin its own execution. Asynchronous Operation - In this case the 2nd process does not wait on the completion of process 1, instead it continues with its own execution, if it need anything from process 1, it executes its callback functions (promise concept) when process 1 has the result available.