Using async/await with a forEach loop
Sure the code does work, but I'm pretty sure it doesn't do what you expect it to do. It just fires off multiple asynchronous calls, but the printFiles
function does immediately return after that.
Reading in sequence
If you want to read the files in sequence, you cannot use forEach
indeed. Just use a modern for … of
loop instead, in which await
will work as expected:
async function printFiles () {
const files = await getFilePaths();
for (const file of files) {
const contents = await fs.readFile(file, 'utf8');
console.log(contents);
}
}
Reading in parallel
If you want to read the files in parallel, you cannot use forEach
indeed. Each of the async
callback function calls does return a promise, but you're throwing them away instead of awaiting them. Just use map
instead, and you can await the array of promises that you'll get with Promise.all
:
async function printFiles () {
const files = await getFilePaths();
await Promise.all(files.map(async (file) => {
const contents = await fs.readFile(file, 'utf8')
console.log(contents)
}));
}
With ES2018, you are able to greatly simplify all of the above answers to:
async function printFiles () {
const files = await getFilePaths()
for await (const contents of files.map(file => fs.readFile(file, 'utf8'))) {
console.log(contents)
}
}
See spec: proposal-async-iteration
Simplified:
for await (const results of array) {
await longRunningTask()
}
console.log('I will wait')
2018-09-10: This answer has been getting a lot of attention recently, please see Axel Rauschmayer's blog post for further information about asynchronous iteration.