A simple utility for Node.js to wait until a predicate function returns truthy before moving on; for use with ES6 async/await syntax.

until.js

/**
 * Utility that waits for @predicate function to return truthy, testing at @interval until @timeout is reached.
 *
 * Example: await until(() => spy.called);
 *
 * @param {Function} predicate
 * @param {Number} interval
 * @param {Number} timeout
 *
 * @return {Promise}
 */
async function until(predicate, interval = 500, timeout = 30 * 1000) {
  const start = Date.now();

  let done = false;

  do {
    if (predicate()) {
      done = true;
    } else if (Date.now() > (start + timeout)) {
      throw new Error(`Timed out waiting for predicate to return true after ${timeout}ms.`);
    }

    await new Promise((resolve) => setTimeout(resolve, interval));
  } while (done !== true);
}

module.exports = until;