JavaScript Synchronous vs Asynchronous Programming

 Synchronous JavaScript

        synchronous means to be in a sequence, i.e. every statement of the code gets executed one by one. So, basically a statement has to wait for the earlier statement to get executed.

        A synchronous request blocks the client until operation completes. i.e. browser is unresponsive in such case, javascript engine of the browser is blocked.

   console.log('Starting')
   alert('pending');
   console.log('end')

Output

Starting
//show alert('pending') then
end

What is that means, JavaScript is a single threaded synchronous programming language. It not allow multiple thing at a time. js code execute line by line. supposed consider this alert, if we are not click the 'ok' button, so next function are not executed, it will stoped. i click 'ok' button then the next line will executed. 

Asynchronous JavaScript

    Asynchronous code allows the program to be executed immediately where the synchronous code will block further execution of the remaining code until it finishes the current one.

    |An asynchronous request doesn't block the client. i.e. browser is responsive.    

    At that time, user can perform another operations also. In such case, javascript engine of the browser is not blocked.

setTimeout Method is used to fun2(); exicute after 3sec.

        console.log('Starting')
        setTimeout(()=>{
            alert('pending');
        }, 5000);
        console.log('end')

Output

Starting
end
//then it will show alert