It means that it doesn't wait for the response of an API call, I/O events, etc., and can continue the code execution. It is possible thanks to the JS engines which use (under the hood) real multi-threading languages, like C++ (Chrome) or Rust (Firefox). 
rev2022.7.19.42626. JavaScript is a single-threaded language because while running code on a single thread, it can be really easy to implement as we dont have to deal with the complicated scenarios that arise in the multi-threaded environment like deadlock. You can always change the cookie settings in your browser. It has to do this because each line depends on the work done in the preceding lines. Similarly, within the call stack, whenever a line of code gets inside the call stack it gets executed and move out of the stack. 
Why split the