Why is my async/await implementation not wokring?

Question

I have a list of csv-files and want to parse them. For this I imported fast-csv. I tried to implement the functionality as described in another stack overflow thread.

But in my code it does not work, and I am really confused why it does not work. I always get an empty array as output for log(data).

import { ParserOptionsArgs, parseFile } from '@fast-csv/parse'
import fs from 'fs'
import path from 'path'

var files: string[] = []
fs.readdirSync(inArg).forEach((file) => {
  files.push(path.parse(file).name)
})

files.forEach( async(file) => {
    const data: any = await readCsv(`${inArg}/${file}.csv`, { headers: true }, (row) => {})
    log(data)
    // here I want make some useful with the data
})

function readCsv(path: string, options: ParserOptionsArgs | undefined, rowProcessor: (arg0: any) => any) {
  return new Promise((resolve) => {
    const data: any = []

    parseFile(path, options)
      .on('data', (row: any) => {
        const obj = rowProcessor(row)
        if (obj) {
          data.push(obj)
          log(obj)
        }
      })
      .on('end', () => {
        resolve(data)
        log('Done')
      })
  })
}

The console output:

Files: [ 'test1', 'test2', 'test3', 'test4' ]
Done
[]
Done
[]
Done
[]
Done
[]

May someone can help me out?

derpirscher · Accepted Answer · 2023-01-30T21:02:19.017

Your async/await code is fine. BUT: You are passing a rowProcessor function of

(row) => {}

to your readCsv. This rowProcessor doesn't return anything. I know it's a subtle pitfall, I stumbled upon this more than once. But how it's written above, the {} does not define an object, but an empty function body. If you really intend to return an empty object your rowProcessor should be

(row) => { return {}; }

or

(row) => ({})

Now, as your rowprocessor doesn't return anything, in the following snippet

const obj = rowProcessor(row)
if (obj) {
  data.push(obj)
  log(obj)
}

obj will of course always be undefined thus, nothing is pushed to your data array and hence log(data) will log the empty array, we see.

Thank you very much. That's the problem when you take over stackoverflow snippets of things you don't really understand yet. I have now just removed the `rowProcessor` completely and return the `row` directly because I don't need it and it does exactly as I expected. — Elmar B., Jan 30 '23 at 21:02

Why is my async/await implementation not wokring?

1 Answers1