async.whilst

This modules provides a functionality of a generalized asynchronous loop. It takes up to three arguments:

• pred() — a predicate function (possibly asynchronous) that for a given value returns a truthy or falsy value. A truthy value continues a loop, while a falsy value stops it, and resolves a final promise to the last value.
• body() — an optional function (possibly asynchronous) that takes a value, and returns another (or same) value. If it is not specified or falsy, the result of pred() is going to be used as a result of an interation.
• Deferred — an optional deferred constructor for internal promises (realistically Deferred or FastDeferred), or a falsy value to use the standard Promise.

It returns a function that takes one argument (an arbitrary value) to start an asynchronous loop, and returns a promise.

Example (with synchronous functions for simplicity):

var whilst = require('heya-async/whilst');

// calculate a factorial
var loop = whilst(
  function (tuple) { // pred
    return tuple[0] > 0;
  },
  function (tuple) { // body
    return [tuple[0] - 1, tuple[1] * tuple[0]];
  }
);

loop([6, 1]).then(function (tuple) {
  console.log(tuple[0] + '! = ' + tuple[1]); return tuple;
});