how can await cause a for loop to exit - node.js

Question

I've been having an issue with awaiting a function in a for loop in Nodejs
Initially I believed that I was missing something in the stream docs but then rewrote my code to not use streams and still have the same issue
It seems as though using await in a for loop is causing the loop to wait once till the promise resolves then exits the loop
here's an example of my issue: it uses streams as its simpler but I can post a longer example without them if needed
const { PassThrough } = require("stream");
const waitTillStreamEnd = (stream) => {
console.log("waiting for end");
const prom = new Promise((res) => {
stream.on("end", res);
});
return prom.then((e) => {
console.log(e);
return e;
});
};
(async () => {
for (let i = 0; i < 10; i++) {
const stream = new PassThrough();
setTimeout(() => stream.end("test"), 3000);
console.log("timeout set");
const res = await waitTillStreamEnd(stream);
console.log("after await");
}
})();
my output from this is:
timeout set
waiting for end
Nothing after the promise resolves is executed, removing the await causes the loop to run 10 times as I'd expect but it only ones once with the await statement there
Any help would be appreciated

Answer 1

It looks like the promise that you set up in waitTillStreamEnd is not being resolved, which means your loop is not going to get past the first execution.
I note that in waitTillStreamEnd you are returning a promise.then()
However, you should just return the promise, so that it can be properly resolved by the await call in your loop.
Also, with this code:
then((e) => {
console.log(e);
return e;
});
Looks like you are trying to do some error handling with this, but it's not working.
A better place to do error handling would be by providing a second reject callback when setting up your promise, and then listening for the stream error event. This would then cause the error to be propagated through to your for loop for proper handling via a try catch block.
Something along these lines, just typed this on the fly, so there may be some formatting issues :
const { PassThrough } = require("stream");
const waitTillStreamEnd = (stream) => {
console.log("waiting for end");
const prom = new Promise((resolve, reject) => {
stream.on("end", resolve);
stream.on("error", err=>{
reject(err) });
});
return prom;
};
(async () => {
try {
for (let i = 0; i < 10; i++) {
const stream = new PassThrough();
setTimeout(() => stream.end("test"), 3000);
console.log("timeout set");
const res = await waitTillStreamEnd(stream);
console.log("after await");
}
} catch(err) {
console.error(err);
}
})();
Good luck !