loop-write.js

import { open, writeFile } from 'fs';

/**
 * Writes and overwrites a file in a continuous loop.
 *
 * Example:
 *     loopWriteFile('./date.txt', function*() {
 *         while (true) {
 *             yield new Date().toISOString();
 *         }
 *     }).catch(e => {
 *         console.error('Done');
 *     });
 * 
 * @param  {(string|Buffer)}    filename    The path to the file to write to.
 * @param  {()=> Iterator<any>} generator   The callback that generates the data to be written.
 * @param  {number}             [time = 0]  The time between intervals.
 * @return {Promise<any>}
 */
export default function loopWriteFile(filename, generator, time = 0) {
  return new Promise(function(resolve, reject) {
    /** @type {number?} */
    let timeout = null;
    let start = Date.now();
    open(filename, 'w', function(/** @type {Error?} */ error, file) {
      let g = generator();
      iterate();

      function next(/** @type {Error?} */ e) {
        error = e;
        let now = Date.now();
        timeout = setTimeout(iterate, Math.max(time + start - now, 0));
        start = now;
      }

      function iterate() {
        try {
          let call = error
          ? g.throw(error)
          : g.next();
          if (!call.done) {
            Promise.resolve(call.value).then(function(value) {
              writeFile(file, value == null ? '' : value, function(e) {
                if (e) {
                  next(e);
                } else {
                  let d = new Date();
                  fs.utimes(filename, d, d, next);
                }
              });
            }).catch(next);
          } else {
            resolve(call.value);
          }
        } catch (e) {
          reject(e);
        }
      }
    });
  });
};