debounce.js

/**
 * @example
 *   let refetch = debounce((params) => makeRequest('/api', params));
 *   let prA = refetch(paramsA);
 *   let prB = refetch(paramsB);
 *   let prC = refetch(paramsC);
 *   let prD = refetch(paramsD);
 *   (prA === prB === prC === prD)
 *   awaits the last promise
 */

function debounce(fn) {
  let head = null;
  let current = null;
  return (...args) => {
    let promise = fn(...args);
    current = promise;
    if (!head) {
      head = deferred();
    }
    promise.then(
      value => {
        if (current === promise) {
          head.resolve(value);
          head = null;
        }
      },
      error => {
        if (current === promise) {
          head.reject(error);
          head = null;
        }
      }
    );
    return head.promise;
  };
}

function deferred() {
  let resolve = null;
  let reject = null;
  let promise = new Promise((rslv, rjct) => {
    resolve = rslv;
    reject = rjct;
  });
  return { promise, resolve, reject };
}