CSV I/O

CSV I/O (csvio) is a Node.js library for processing CSV files.

Code repository mirrors: GitHub, Codeberg, git.ardis.lu

Install

npm i @ardislu/csvio

(OPTIONAL) If you are using TypeScript or you are type-checking JavaScript, also install @types/node:

npm i -D @types/node

Example

Suppose you have a ./in.csv file that looks like:

And you want to create a new ./out.csv file that looks like:

To do this, run:

import { CSVReader, CSVTransformer, CSVWriter } from '@ardislu/csvio';

function calculate(row) { // Input is a simple array representing one CSV row
  const [country, id] = row[0].split('-');
  const total = (Number(row[1]) * Number(row[2])).toFixed(2);
  return [country, id, total]; // Output is also a simple array
}

// Specify output headers with a simple array
const headers = ['Country', 'ID', 'Total'];

// Use web standard Streams API
await new CSVReader('./in.csv') // Automatically handles path-like input
  .pipeThrough(new CSVTransformer(calculate, { handleHeaders: headers }))
  .pipeTo(new CSVWriter('./out.csv'));

See the examples folder for more end-to-end examples.

Why?

A common workflow is:

1. Read an existing input CSV file.
2. Perform some processing using the data in the input CSV file.
3. Write data to an output CSV file.

Most people are interested in coding the processing logic for step (2) and less interested in coding the boilerplate for steps (1) and (3).

CSV I/O provides minimal code optimized to accomplish this specific workflow. This library is not intended for any other workflow (e.g., parsing CSV files with custom delimiters, converting between CSV and other formats, generating CSV data). For those workflows, see "Other CSV libraries".

Features

• No dependencies other than the Node.js standard library.
• Reads and writes RFC 4180 compliant CSV files.
• Optimized for common CSV processing requirements (e.g., concurrent processing, batching, error handling).
• Single core.js file contains all core API code.
• Prioritizes WinterTC (TC55) Minimum Common Web Platform APIs (e.g., the Web Streams API over Node.js Streams).

Core API

The core API is contained in src/core.js and exposes three classes to read, transform, and write CSV files.

CSVReader

A ReadableStream where each chunk is one row from a local CSV file.

import { CSVReader } from '@ardislu/csvio';

const readableStream = new CSVReader('./examples/data/ex1_1-in.csv');

for await (const row of readableStream) {
  console.log(row);
}
// [ 'columnA', 'columnB' ]
// [ '1', '1' ]
// [ '100', '100' ]
// [ '223423', '455947' ]
// [ '348553', '692708' ]
// [ '536368', '676147' ]

CSVTransformer

A TransformStream that will apply a given function to each row in a streamed CSV.

import { CSVTransformer } from '@ardislu/csvio';

function edit(row) {
  return [`${row[0]}: transformed!`];
}

const transformStream = new CSVTransformer(edit);

const rows = ReadableStream.from((function* () {
  yield ['header'];
  yield ['1'];
  yield ['2'];
  yield ['3'];
})()).pipeThrough(transformStream);

for await (const row of rows) {
  console.log(row);
}
// [ 'header' ]
// [ '1: transformed!' ]
// [ '2: transformed!' ]
// [ '3: transformed!' ]

CSVWriter

A WritableStream to save CSV row data to disk.

import { CSVWriter } from '@ardislu/csvio';

const writableStream = new CSVWriter('./example.csv');

await ReadableStream.from((function* () {
  yield ['header1', 'header2'];
  yield ['1', '1'];
  yield ['2', '2'];
  yield ['3', '3'];
})()).pipeTo(writableStream);
// A new ./example.csv file is created with the CSV data

Extended API

In addition to the core API, optional utilities are provided to address common CSV transformation tasks.

CSVNormalizer and CSVDenormalizer

CSVNormalizer is a TransformStream that applies opinionated transformations to correct data mangling caused by spreadsheet programs and manual user manipulation:

• Remove empty or extra data: Spreadsheet programs may append empty rows or columns at the ends of a CSV file, or users may insert empty rows or columns for formatting purposes. These empty rows are removed. Additionally, columns which are not explicitly listed in the CSVNormalizerHeader array are also removed. Note that empty rows that come before the header row are always removed, even if you have set passthroughEmptyRows: true.
• Fix date and number mangling: Spreadsheet programs may automatically cast data types in unexpected ways. CSVNormalizer will perform various transformations to reverse data type casting that was unlikely to be intentional.
• Flexible column naming: Column names are normalized to camelCase to prevent issues with subtly inconsistent names (e.g., Column A, column A, and ColumnA are all considered to be the same name).
• Column ordering: Columns may appear in any order on the input CSV. The columns in the output are re-ordered to match the order in the CSVNormalizerHeader array.
• Stream of objects: The stream is transformed from an Array<string> to an Array<CSVNormalizerField> to provide a better developer experience for subsequent transformations.

CSVDenormalizer is a TransformStream that will convert an Array<CSVNormalizerField> created by CSVNormalizer back into an Array<string>, ready for writing to a file.

import { CSVNormalizer, CSVDenormalizer } from '@ardislu/csvio';

const headers = [
  { name: 'numberCol', displayName: 'Number Column', type: 'number' },
  { name: 'dateCol', displayName: 'Date Column', type: 'date' }
]

const rows = ReadableStream.from((function* () {
  yield ['', '', '', '', '']; // Empty row inserted before the header row
  yield ['', 'numberCol', 'dateCol', '', '']; // Empty columns inserted, including before the data
  yield ['', '1E+18', '45292', '', '']; // Number mangled to scientific notation; date mangled to number
  yield ['', '', '', '', '']; // Empty rows inserted
  yield ['', '', '', '', ''];
  yield ['', '', '', '', ''];
})())
  .pipeThrough(new CSVNormalizer(headers))
  .pipeThrough(new CSVDenormalizer());

for await (const row of rows) {
  console.log(row);
}
// [ 'Number Column', 'Date Column' ]
// [ 1000000000000000000, '2024-01-01T00:00:00.000Z' ]

Tests

All example and test CSVs (except for lf-eol.csv) use CRLF (\r\n) as the record delimiter, as suggested by RFC 4180.

If your IDE is configured to automatically convert CRLF line endings to LF, you must undo this conversion for the CSV files inside /test/data and /examples/data.

Run primary tests:

npm test

Run coverage report for primary tests:

npm run test:coverage

Run large.test.js tests (writing, reading, and transforming ~100 MB CSV files). These tests take longer to complete, so they are separated from the primary tests:

npm run test:large

Other CSV libraries

Here are other CSV libraries that may be better for your use case.

• Papa Parse
• node-csv
• fast-csv
• csv-parser