An idea for how generators could be implemented pre-ES6

createGeneratorIdea.js

/**
 * Using yield to create a generator -- only possible in ES6/Harmony.
 */
function fibonacci(limit) {
  var fn1 = 1;
  var fn2 = 1;
  while (1) {
    var current = fn2;
    fn2 = fn1;
    fn1 = fn1 + current;
    if (limit && current > limit)
      return;
    yield current;
  }
}

/**
 * This should be possible in pre-ES6 versions of ECMAScript.
 */
var fibonacci = createGenerator(function(limit, yield) {
  var fn1 = 1;
  var fn2 = 1;
  while (1) {
    var current = fn2;
    fn2 = fn1;
    fn1 = fn1 + current;
    if (limit && current > limit)
      return;
    yield(current);
  }
});

/**
 * How would it work?
 *
 * - generator uses a JS parser (e.g., esprima, acorn) to parse the source of
 *   the function passed to it using Function.prototype.toString.
 *
 * - all possible loops (for, while, do/while) are translated in the AST to
 *   async-friendly forms accepting callbacks.
 *
 *   for example:
 *   while (1) { [code] }
 *
 *   would become:
 *   library.while(function() { return 1; }, function() { [code] });
 *
 *   these async-friendly loop forms would be implemented by this library
 *
 * - yield statements convert values to functions in a similar way, so:
 *
 *   yield(current);
 *
 *   would become:
 *   yield(function() { return current; });
 *
 * - the library would then generate code (using something like escodegen) to
 *   generate the modified code from the AST and construct a new Function object
 *   from the resulting code
 *
 * - this function would return a generator with the same interface as
 *   generators in ES6
 */