JavaScript is a single-threaded language, which means it has only one call stack that is used to execute the program. The call stack is the same as the stack data structure that you might read in Data structures. As we know stacks are FILO that is First In Last Out. Similarly, within the call stack, whenever a line of code gets inside the call stack it gets executed and moves out of the stack. In this way, JavaScript is a single-thread language because of only one call stack. JavaScript is a single-threaded language because while running code on a single thread, it can be really easy to implement as we don't have to deal with the complicated scenarios that arise in the multi-threaded environment like a deadlock.