Asynchronous Code using Timers

Asynchronous Code Using Timers

Synchronous code is run line by line in the order in which the code occurred.

Notice how synchronous code is executed:

Asynchronous code may be executed in a different order than how it originally occurred. Asynchronous code is non-blocking and will only run when the call stack is empty.

Asynchronous code can be shown by using a setTimeout() method call with a timeout value of 0. This will immediately put a task on the event queue.

Notice how "second" is logged asynchronously and occurs out of order:

The output appears out of order because the asynchronous console log task had to wait for the call stack to finish executing the other console logs before it could occur.