Throttle an async (promise-returning) function in JavaScript/Node.js

throttle_async.js

// Throttles an async function.
function throttleAsync(fn, wait) {
  let lastRun    = 0;
  let currentRun = null;

  async function throttled(...args) {
    if (currentRun)
      return currentRun;

    const currentWait = lastRun + wait - Date.now();
    const shouldRun   = currentWait <= 0;

    if (shouldRun) {
      lastRun     = Date.now();
      currentRun  = fn(...args);
      const value = await currentRun;
      currentRun  = null;
      return value;
    } else {
      return await new Promise(function(resolve) {
        setTimeout(function() {
          resolve(throttled(...args));
        }, currentWait);
      });
    }
  }

  return throttled;
}