NodeJS: Parallelism of async module
async.js, node.js
Solution
The 'parallel' in the async documentation doesn't refer to 'parallel' in terms of concurrency (like multiple processes or threads being run at the same time), but 'parallel' in terms of each step being independent of the other steps (the opposite operation would be `eachSeries`, where each step is run only after the previous has finished).
The parallel version would only make sense if the steps perform some kind of I/O, which (because of Node's asynchronous nature) could run parallel to each other: if one step has to wait for I/O, the other steps can happily continue to send/receive data.
If the steps are mainly cpu-bound (that is, performing lots of calculations), it's not going to provide you any better performance because, like you say, Node runs the interpreter in a single thread, and that's not something that `async` changes.
Problem
I am following the async module's each method (https://github.com/caolan/async#each). It says the method iterates over the array parallely. "Parallely" is the word that confuses me. AFAIK, in now way JavaScript can execute code parallely because it has a single-threaded model. The examples shown in the each method focuses on the IO scenarios. I am using the "each" method just to add numbers of the array. If parallelism exists, can I prove this using my example? Thanks for reading.