interject synchronous code into asynchoronous

Question

I'm running the lighthouse cli against a ~50 site list of sites. I'm simply running it in a .forEach loop, which, if I understand, is blocking, aka, synchronous. However, I end up spinning up 50 Chrome Canary instances all at once. In my limited understanding of these things I think the thread is started synchronously, but then node can pass the thread off to the kernel and happily start the next. Again, that's just my hand-wavy understanding of what's going on.

I'm using this function that I cribbed from somewhere:

function launchChromeAndLighthouse(url, opts, config = null) {
  return chromeLauncher.launch({chromeFlags: opts.chromeFlags}).then(chrome => {
    opts.port = chrome.port;
    return lighthouse(url, opts, config).then(results =>
      chrome.kill().then(() => results));
  });
}

I tried nextTick in the loop:

asyncFuncs().then( async (sites) => {
  sites.forEach( (site) => {
    process.nextTick(launchChromeAndRunLighthouse(site.url, opts))
  })
})

But this still spawns a bunch of Chrome instances. How do I pause execution while one lighthouse completes?

Answer 1

Since launchChromeAndRunLighthouse() returns a promise to mark when it's done, if you want to only run them serially one at a time, you can switch to a for loop and use await:

asyncFuncs().then( async (sites) => {
  for (let site of sites) {
    await launchChromeAndRunLighthouse(site.url, opts);
  }
});

If you're trying to collect all the results:

asyncFuncs().then( async (sites) => {
    let results = [];
    for (let site of sites) {
      let r = await launchChromeAndRunLighthouse(site.url, opts);
      results.push(r);
    }
    return results;
}).then(results => {
    // all results here
}).catch(err => {
    // process error here
});

---

If you want to run N chrome instances at a time such that it initially starts up N instances and then each time one finishes, you launch the next one that's waiting, that is more complicated to keep track of how many instances are running. There's a helper function call pMap() or mapConcurrent() that can do that for you in these answers:

Make several requests to an API that can only handle 20 request a minute

Promise.all consumes all my RAM

---

The Bluebird Promise library also has concurrency control in its Promise.map() function.