Promise retry method for functions returning a thenable interface

function-promise-retry.js

Function.prototype.retryWhile = function(keepTrying) {
  var target = this;
  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:
something.retryWhile(function(e, n) { return n<5; }).then(progress, error)
 
target.retryWhile(keepTrying) -> promise:
Return a promise that is fulfilled when the promise returned by target is fulfilled. It keeps trying until test
function is false. Each retry, keepTrying function receives the reject value and the number of retry.
 
*/