events can be scheduled while code is running, but can't be executed until the runtime is free

timeouts.js

var start = new Date();

setTimeout(function () { var end = new Date(); console.log("Timeout 3ms elapsed:", end - start, "ms"); }, 3);
setTimeout(function () { var end = new Date(); console.log("Timeout 2ms elapsed:", end - start, "ms"); }, 2);
setTimeout(function () { var end = new Date(); console.log("Timeout 1ms elapsed:", end - start, "ms"); }, 1);

while( new Date() - start < 2000 ) {}

setTimeout(function () { var end = new Date(); console.log("Timeout 0ms elapsed:", end - start, "ms"); }, 0);

// Timeout 1ms elapsed: 2000 ms
// Timeout 2ms elapsed: 2000 ms
// Timeout 3ms elapsed: 2000 ms
// Timeout 0ms elapsed: 2001 ms