How to wait for all Promises to be finished

November 24, 2021

I like to run a set of functions in parallel and when all are done, I like to run some final stuff. My JavaScript looks like this:

const logger = console;

const functionOne = function () {
  logger.info("Starting functionOne");
  return new Promise(resolve => {
    setTimeout(function () {
      logger.info("Finished functionOne after 20 sec.");
    }, 20000);
  });
};
    
const functionTwo = function () {
  logger.info("Starting functionTwo");
  return new Promise(resolve => {
    setTimeout(function () {
      logger.info("Finished functionTwo after 10 sec.");
    }, 10000);
  });
};

const runningFunctions = async function () {
  logger.info('Start jobs');
  functionOne();
  functionTwo();
}

runningFunctions();
logger.info(`All done after 20 sec.`);

In principle my log output should be this one:

2021-11-24 16:54:31.111 [info] -> Start jobs
2021-11-24 16:54:31.112 [info] -> Starting functionOne
2021-11-24 16:54:31.113 [info] -> Starting functionTwo
2021-11-24 16:54:41.115 [info] -> Finished functionTwo after 10 sec.
2021-11-24 16:54:51.115 [info] -> Finished functionOne after 20 sec.
2021-11-24 16:54:51.116 [info] -> All done after 20 sec.

I tried different solution, e.g.

runningFunctions().then(
  () => { logger.info(`All done after 20 sec.`) }
).catch(
  err => { logger.error(err) }
);

Promise.all([functionOne(), functionTwo()]).then(() => {
    logger.info(`All done after 20 sec.`);
});

and many others, but none of them worked as expected. In most cases message All done after 20 sec. comes immediately after functionOne/functionTwo started or the final log is not printed at all.

How do I have to write the script?

>Solution :

Promise.all is the way to go, but you’d need to resolve your promises

const logger = console;

const functionOne = function () {
  logger.info("Starting functionOne");
  return new Promise(resolve => {
    setTimeout(function () {
      logger.info("Finished functionOne after 20 sec.");
      resolve(); // need to resolve
    }, 20000);
  });
};
    
const functionTwo = function () {
  logger.info("Starting functionTwo");
  return new Promise(resolve => {
    setTimeout(function () {
      logger.info("Finished functionTwo after 10 sec.");
      resolve();
    }, 10000);
  });
};

Promise.all([functionOne(), functionTwo()]).then(() => {
    logger.info(`All done after 20 sec.`);
});