Promise retry function with thenable interface

promise-retry.js

function retry(target, keepTrying) {
  var retryCount = 1;
  var fulfill, reject;

  var promise = new Promise(function (f, r) {
    fulfill = f;
    reject = r;
  });
  
  realRetry();
  return promise;
  
  function realRetry() {
    target().then(
      function (result) { fulfill(result); },
      function (error) { setTimeout(
        function _doRetry() {
          if (keepTrying(error, retryCount)) {
            retryCount++;
            realRetry();
          } else {
            reject(error);
          }
        }
      ); }
    );
  }
}

/*
How to use:
  - Retry something().then(progress, error) 5 times:
      retry(something, function(e, n) { return n<5; }).then(progress, error)

function retry(target, keepTrying) -> promise:
  It takes a target function returning a promise and a test function and keep trying until test function is false.
  Each retry, keepTrying function receives the reject value and the number of retry.

*/