Demonstration of how setTimeout and multi-thread work in JS. Read the comment for more details.

multi_thread.js

// original reference https://davidwalsh.name/es6-generators

/**
 * This function create a timeout that should be executed after 1 millisecond
 */
function timeoutFunction() {
  setTimeout(function() {
    console.log('Timeout');
  }, 1);
}

/**
 * This function create a a loop that takes more than 1 millisecond to be 
 * executed
 */
function loopFunction() {
  for (var i = 0; i < 1000; i++) {
    console.log('Loop');
  }
}

/**
 * This Self-Invoking function make sure that the two previous function are 
 * called one right after the other
 */
(function() {
    timeoutFunction();
    loopFunction();
})();

First, we create a timeout at 1 millisecond, then we execute a function that lasts longer than 1 millisecond.

In a multi-thread environment we will expect to see in the console the output of the timeout exactly after 1 millisecond, but javascript is single-threaded so we need to wait until the second function is over before executing the timeout function.

So setTimeout shouldn't be read as "Execute this function after X millisecond" but as "Wait X millisecond, and then at the first available time execute this function".

thanks for explanation