Node.js fast-csv synchronously read file and return array

November 24, 2020, at 7:50 PM

I am trying to read a .csv file and to assign it to an array with this code:

const fs = require("fs");
const csv = require("@fast-csv/parse");
let data = []
csv
  .parseFile("./downloads/aggiornamento.csv", { skipRows: 2 })
  .on("error", (error) => console.error(error))
  .on("data", (row) => {
    let obj = {};
    obj["item_id"] = row[2];
    obj["qty"] = row[20];
    data.push(obj);
    console.log(obj);
  })
  .on("end", function () {
    return data;
  });

However, the file read by fast-csv is a stream, hence I can only process row by row. I need the full array in a second moment to call some API service, hence I would like to read the file synchronously, appending very row, and returning the array at the end, so that, in a second moment, I can wrap the previous code in a function, and obtain the array in a way such as:

const data = readCSVSynchronously()

How can I do it with fast-csv?

Answer 1

You can't turn an asynchronous function into a synchronous one.

You could, however, wrap that eventful reading code into a promise, which you can then use in a promise/async context, like so:

const fs = require("fs");
const csv = require("@fast-csv/parse");
function readCsv(path, options, rowProcessor) {
  return new Promise((resolve, reject) => {
    const data = [];
    csv
      .parseFile(path, options)
      .on("error", reject)
      .on("data", (row) => {
        const obj = rowProcessor(row);
        if (obj) data.push(obj);
      })
      .on("end", () => {
        resolve(data);
      });
  });
}
async function doThings() {
  const data = await readCsv(
    "./downloads/aggiornamento.csv",
    { skipRows: 2 },
    (row) => ({ item_id: row[2], qty: row[20] }),
  );
  // use data in API...
}

Answer 2

Callback functions can be wrapped with promise.

const fs = require("fs");
const csv = require("@fast-csv/parse");
let data = []
let readCSVSynchronously = async () = {
    return new Promise(reject, resolve => {
     const results = [];
     csv
      .parseFile("./downloads/aggiornamento.csv", { skipRows: 2 })
      .on("error", (error) => console.error(error)
          return reject(error);
       )
      .on("data", (row) => {
        // Do your logic
        results.push(row);
      })
      .on("end", function () {
          resolve(results)
      });
    });
}
const totalResults = await readCSVSynchronously();