Node Synchronous loops - node.js

Question

I've got a function.
function async extractTars (tarList) {
try {
for (let i = 0; i < tarList.length; i ++) {
// I need this loop to be sync but it isn't hitting the next iteration
return new Promise((resolve, reject) => {
fs.createReadStream(`${PATHS.TAR}/${tarList[i]}`)
.pipe(tar.extract(PATHS.GZ))
.on('error', err => reject(err))
.on('finish', () => resolve())
})
}
// What is the correct way to resolve this async fn? Should i just return or will it resolve anyway after the loop?
} catch (e) {
// handle error
}
}
For some reason it never hits the next iteration of the loop. What am i missing here? How can i make these type of loops synchronous. In this particular project i've got many many loops that need to complete before the next. I've tried several methods that i've see here on SO but i'm obviously missing something.
Any help would be much appreciated!

Answer 1

You're using return, that's why you're not hitting the next iteration, use await instead to wait until the promise is done before going to the next iteration.
return inside a for will stop the loop & end the function returning the new Promise
function async extractTars (tarList) {
try {
for (let i = 0; i < tarList.length; i ++) {
// I need this loop to be sync but it isn't hitting the next iteration
await new Promise((resolve, reject) => {
fs.createReadStream(`${PATHS.TAR}/${tarList[i]}`)
.pipe(tar.extract(PATHS.GZ))
.on('error', err => reject(err))
.on('finish', () => resolve())
})
}
// What is the correct way to resolve this async fn? Should i just return or will it resolve anyway after the loop?
} catch (e) {
// handle error
}
}
And just to clear things up:
I need this loop to be sync but it isn't hitting the next iteration
That loop won't be sync, it will look like it's sync, when using async/await but you can't make asynchronous code be synchronous. And createReadStream is asnychronous.
And regarding:
What is the correct way to resolve this async fn? Should i just
return or will it resolve anyway after the loop?
When all iterations are done or an error is thrown (since you're catching it and not rejecting), meaning that you read all the files in tarList, the function will be resolved. Since you are not returning anything, undefined will be the resolved value.
From MDN:
When a return statement is used in a function body, the execution of the function is stopped. If specified, a given value is returned to the function caller. For example, the following function returns the square of its argument, x, where x is a number.