Throttling-promises.js

/**
 * Performs a list of callable actions (promise factories) so that only a limited
 * number of promises are pending at any given time.
 *
 * @param listOfCallableActions An array of callable functions, which should
 *     return promises.
 * @param limit The maximum number of promises to have pending at once.
 * @returns A Promise that resolves to the full list of values when everything is done.
 */
function throttleActions(listOfCallableActions, limit) {
  // We'll need to store which is the next promise in the list.
  let i = 0;
  let resultArray = new Array(listOfCallableActions.length);

  // Now define what happens when any of the actions completes. Javascript is
  // (mostly) single-threaded, so only one completion handler will call at a
  // given time. Because we return doNextAction, the Promise chain continues as
  // long as there's an action left in the list.
  function doNextAction() {
    if (i < listOfCallableActions.length) {
      // Save the current value of i, so we can put the result in the right place
      let actionIndex = i++;
      let nextAction = listOfCallableActions[actionIndex];
      return Promise.resolve(nextAction())
          .then(result => {  // Save results to the correct array index.
             resultArray[actionIndex] = result;
             return;
          }).then(doNextAction);
    }
  }

  // Now start up the original <limit> number of promises.
  // i advances in calls to doNextAction.
  let listOfPromises = [];
  while (i < limit && i < listOfCallableActions.length) {
    listOfPromises.push(doNextAction());
  }
  return Promise.all(listOfPromises).then(() => resultArray);
}

// Test harness:

function delay(name, ms) {
  return new Promise((resolve, reject) => setTimeout(function() {
    console.log(name);
    resolve(name);
  }, ms));
}

var ps = [];
for (let i = 0; i < 10; i++) {
  ps.push(() => delay("promise " + i, Math.random() * 3000));
}

throttleActions(ps, 3).then(result => console.log(result));