Promise Gating

gate.js

/**
 * Create a "gated" (throttled) version of the user function. This throttling is not time
 * based but rather throttles execution to prevent two async actions occuring in parallel.
 * Ie. Two syncronous calls to the returned function will return reference to the same promise
 * - An example use case of this would be to prevent firing too many api requests to the same
 *   endpoint in a short amount of time
 * @param {Function} func func *must* return a promise
 * @returns {Function} throttled function which will always return a promise
 */
export function gate(func) {
  let lastPromise = null;

  return function callUserFunc(...args) {
    if (!lastPromise) {
      // Track promise result from func call and
      // hold onto it until the promise is resolved
      lastPromise = func(...args).then(
        function reset() {
          lastPromise = null;
        },
        function resetAndThrow(err) {
          lastPromise = null;
          throw err;
        }
      );
    }

    return lastPromise;
  };
}