async.js

// Synchronous
var a = function (x) {
  while(new Date().getTime() < now + 1000) {  
    // do nothing  
  }
  console.log(x)
};

a(1); // waits 1 sec and prints 1
a(2); // waits for finish `a(1)` then wait another 1 sec and prints 1
a(3); // waits for finish `a(1)` and `a(1)` then wait another 1 sec and prints 1


// Synchronous
var b = function (cb) {
  var start = new Date().getTime();
  while(new Date().getTime() < start + 1000) {  
    // do nothing  
  }
  cb();
};

b(function(){console.log(1)}); // waits 1 sec and prints 1
b(function(){console.log(2)}); // waits for finish `b(function(){console.log(1)});` then wait another 1 sec and prints 1
b(function(){console.log(3)}); // waits for finish `b(function(){console.log(2)});` and `b(function(){console.log(3)});` then wait another 1 sec and prints 1


// Asynchronous
var c = function (cb) {
  var start = new Date().getTime();
  setTimeout(function () {
    while(new Date().getTime() < start + 1000) {  
      // do nothing  
    }
    cb();
  }, 0);
};

c(function(){console.log(1)}); // Init setTimeout's callback and release JS runtime.  
c(function(){console.log(2)}); // Init setTimeout's callback and release JS runtime.  
c(function(){console.log(3)}); // Init setTimeout's callback and release JS runtime.  
// log 1,2,3 at same time.