JavaScript -> synchronous, single-threaded by default

Execution Context -> execute one line of code at a time i.e. each
    operation waits for the last one to complete before executing;
    contains Call Stack and Memory Heap;

Blocking Code -> block the flow of program i.e. read file sync
Non-Blocking Code -> Doesn't block execution i.e. read file async

Event Loop -> to summarise this, let's assume a JS program execution ; creates a Call Stack for function calling and
    a Memory Heap for the variables used ; if the execution is on Web/Browser, then with the help of WebAPI, DOM API can 
    be used and DOM manipulation is done. As the program execution is done, functions stacked in the CallStack space and
    called one by one. 
    For using async nature of JS we use functions like setTimeout(), setInterval() and fetch() to enhance the user engagement
    and efficiency.
    setTimeout() -> executes the program after a specified time, doesn't block the overall execution of the program; 
        registers a CallBack function and creates a TaskQueue that has all the setTimeout program to be executed after the
        time, then it feeds the functions to the CallStack space for execution.
    setInterval() -> executes the program repeatedly after a certain time interval; same execution as of setTimeout

    fetch() -> used to fetch or get any information from the client side or from any API used; returns a Promise;
        creates a TaskQueue which is of high Priority; works in an async manner