understanding generators

gen.js

/**
 * Excerpt from: Analysis of generators and other async patterns in node
 * URL: http://spion.github.io/posts/analysis-generators-and-other-async-patterns-node.html#a-gentle-introduction-to-generators
 *
 * When next is invoked, it starts the execution
 * of the generator. The generator runs until it
 * encounters a yield expression. Then it pauses
 * and the execution goes back to the code that
 * called next.

 * So in a way, yield works similarly to return. 
 * But there is a big difference. If we call next
 * on the generator again, the generator will
 * resume from the point where it left off - 
 * from the last yield line.
 */ 

function *something(msg) {
  msg += yield msg; // yields "a", returns "b"
  msg += yield msg; // yields "ab", returns "c"
  msg += yield msg; // yields "abc", returns "d"
  return msg;       // returns "abcd"
}

var gen = something('a');
console.log(gen.next().value);    // a
console.log(gen.next('b').value); // ab
console.log(gen.next('c').value); // abc
console.log(gen.next('d').value); // abcd