Executing an async function in a loop

Question

I'm writing a function, that loops through port numbers until it finds an open one:

async function findPort (port, app, logger) {

    const lookupPort = () => {
        return portOpen = new Promise( (resolve, reject) => {
            app.listen(port, () => {
                logger.log("info", `Server listening on port: ${port}`);
                resolve(true);
            })
            .on('error', (err) => { 
                logger.log("warn", "port closed: " + err.port);
                reject(false);
            });
        });
    }

    let portOpen = false;

    while (portOpen === false){
        portOpen = await lookupPort();
        port++;
    }

When executed, it iterates through one port and then trows an exception:

(node:2869) UnhandledPromiseRejectionWarning: Unhandled promise rejection.

Answer 1

There are 2 issues with the code above:

1) while loop stops its execution because lookupPort function throws an error (promise is rejected on reject(false); line) which is not caught ( UnhandledPromiseRejectionWarning: Unhandled promise rejection. ). To fix this try to wrap the code inside while loop into try/catch construction:

while (portOpen === false){
  try {
    portOpen = await lookupPort();
  }
  catch (error) {
    // handle an error here
  }
  port++;
}

Or if you do not want to handle the promise you reject inside lookupPort just remove reject(false); line.

2) lookupPort function assigns a new Promise() to portOpen variable. So even if error will be handled by try/catch , portOpen === false condition will evaluate to false because portOpen will be equal to a Promise object. If there is no reason to assin portOpen the value of the new Promise you should remove it.