While JavaScript is a garbage collected language, memory is also allocated natively in C/C++ code. These allocations may occur in the Node.js source code, addons or in the Node.js dependencies (for example V8).
Valgrind is a tool that can be used to instrument an application such that you get reports on memory usage, including potential memory leaks once the application terminates. Valgrind supports Linux as well as some Unix-like variants including macOS.
We recommend using Valgrind to investigate native memory leaks and more detailed information on how to use it with Node.js is available in Investigating Memory Leaks with valgrind