How to create a file stream and write to the stream asynchronously? - node.js

Question

I am new to TypeScript/JavaScript and Node.
Now I am trying to create a file stream and write "Hello!" to the stream asynchronously.
#!/usr/bin/env node
import fs from 'fs';
function createStream(filePath: string): Promise<fs.WriteStream> {
return new Promise<fs.WriteStream>((resolve, reject) => {
const out = fs.createWriteStream(filePath);
out.on('close', () => {
console.log(filePath + ' closed');
resolve(out);
});
out.on('error', (err: any) => {
console.log(filePath + ' ' + err);
reject(err);
});
});
}
createStream('/tmp/test.txt').then((out:fs.WriteStream) => {
console.log(out);
out.write('Hello!');
out.end();
})
This code does create /tmp/test.txt but prints out nothing and the file is empty.
What is the problem with this code ?

Answer 1

You don't need to resolve a promise with the fs.WriteStream since its creation is synchronous. Just call fs.createWriteStream() directly and pass the instance to your function to create a promise that settles when the stream closes or errors:
#!/usr/bin/env node
import fs from 'fs';
import stream from 'stream';
function promisify(s: stream.Stream): Promise<void> {
return new Promise<void>((resolve, reject) => {
const onClose = () => {
s.off('error', onError);
resolve();
};
const onError = (error: Error) => {
s.off('close', onClose);
reject(error);
};
s.once('close', onClose);
s.once('error', onError);
});
}
const out = fs.createWriteStream('/tmp/test.txt');
promisify(out).then(() => {
console.log('Done');
});
out.write('Hello!');
out.end();