Intro

In its most basic form, Javascript is a synchronous, blocking, single-threaded language.

• Synchronous - code executes top-down, with only one line exciting at a time.
• Blocking - subsequent processes won't start until the previous process has finished execution.
• Single-threaded - JS has just one thread called the main thread - for executing code.
• Thread - a simple process that your javascript program can use to run a task, where each thread can only do one task at a time.

Just javascript is not enough to cater to asynchronous programming.

Web browsers help us with this. They define functions and APIs that allow us to register functions that should not be executed synchronously and should be invoked asynchronously when some kind of event occurs.

E.g passage of time (setInterval or setTimeout), user interaction with the mouse (addEventListener), or the arrival of data over the network (callback, Promises, async-await).

Timeouts and Intervals

These are not part of JS, the browser implements them - setTimeout()and setInterval() are names given to that functionality in Javascript.

setTimeout(function,duration, params*) - executes a particular block of code once after a specified time has elapsed.

setInterval(function, duration, params*) - repeatedly runs the same code over and over again, at set intervals.

Note:

• You can clear timeout and intervals using clearTimeout(id) or clearInterval(id) passing in the return value of the setTimeout() or setInterval() function as the id.