Javascript single threaded nature examples below.

single_thread_example.js

// output is 1,3,2
console.log('1');
window.setTimeout(function(){
  console.log('2');
}, 0);
console.log('3');

// because javascript is single threaded, i.e. a block of code is executed in a single thread.
// setTimeout and setInterval are special because they are executed in another thread.
// setTimeout and setInterval send the code to the end of the event bus, they are executed in last.

// so lets take this example, when we have a blocking code like a while loop after setTimeout / setInterval,
// the code of setTimeout / setInterval will execute after the blocking code, for e.g. while loop here.

// also check out this nice article from john resig https://johnresig.com/blog/how-javascript-timers-work/

(function () {

  var startTime = new Date().getTime();

  console.log('setting timeout 0 callback at ' +sinceStart());

  setTimeout(function(){
    console.log('in timeout callback at ' +sinceStart())
  }, 0);

  console.log('starting blocking loop at ' +sinceStart());

  while (sinceStart() < 3000) {
    continue;
  }

  console.log('blocking loop ended at ' +sinceStart());

  return;

  function sinceStart () {
    return new Date().getTime() - startTime
  }

}());

// output is setting 
// timeout 0 callback at 1 debugger eval code:5:3
// starting blocking loop at 1 debugger eval code:11:3
// blocking loop ended at 3000 debugger eval code:17:3
// undefined
// in timeout callback at 3001