read a directory and write the output into a file using streams
Hello,
As a small personal challenge i am trying to use streams for:
1.read a directory (__dirname in here)
2.transform the output as upperCase
3.write the transfomed output into a file
I found this solution, but i doubt that it is the best practice... May someone confirm ? or correct ?
const fs = require('fs') const { Transform, pipeline, Readable } = require('stream') const streamUpper = () => { return new Transform({ transform(chunk, enc, next) { next(null, chunk.toString().toUpperCase()) } }) } async function readDirStream() { const dir = await fs.promises.opendir(__dirname) for await (const dirent of dir) { await require('util').promisify(pipeline)( Readable.from(dirent.name+'\n'), streamUpper(), fs.createWriteStream('./fileList.txt', {flags:'a'}) ) } } readDirStream().catch(e => console.error(e))
Comments
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I just notice that the Readable.from() (line 17) is useless, that is enought
await require('util').promisify(pipeline)( dirent.name+'\n', streamUpper(), fs.createWriteStream('./fileList.txt', {flags:'a'}) )
So it makes me try the course exemple "Reading Directories (.cont)", as there is the same Readable.from() consuming the output from opendir(). And it works without (Readable.from()) as well. I guess it makes sense as opendir() return already a stream, am i right ?
exemple from the course without Readable.from()
opendir(__dirname, (err, dir) => { if (err) { res.statusCode = 500 res.end('Server Error') return } // const dirStream = Readable.from(dir) const entryStream = createEntryStream() res.setHeader('Content-Type', 'application/json') pipeline(dir, entryStream, res, (err) => { if (err) console.error(err) }) })
0 -
- Avoid doing inline
require
- always putrequire
at the top Readable.from
is a convenient way to turn a data structure (string, array) into a readable stream- It's not needed in your directory processing example, and really neither is the transform stream
- A better example to use with your transform stream would be a TCP socket:
const { Transform, pipeline } = require('stream') const streamUpper = () => { return new Transform({ transform(chunk, enc, next) { next(null, chunk.toString().toUpperCase()) } }) } const { createServer } = require('net') createServer((socket) => { pipeline(socket, streamUpper(), socket, (err) => { if (err) console.error(err) else console.log('socket closed') }) }).listen(9999)
Readable.from
would be useful for testing your stream:const { Transform, Readable } = require('stream') const assert = require('assert').strict const streamUpper = () => { return new Transform({ transform(chunk, enc, next) { next(null, chunk.toString().toUpperCase()) } }) } const upper = streamUpper() Readable.from(['test-a', 'test-b']).pipe(upper) upper.once('data', (first) => { assert.equal(first, 'TEST-A') upper.once('data', (second) { assert.equal(first, 'TEST-B') }) })
1 - Avoid doing inline
-
Hello David,
Thank you for the answer, i will work on that. Thank you.
1
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