Why CSV Checker was built

CSV Checker started as a personal project born from a recurring frustration: import failures that only appear after you have already uploaded a file to a production system. The root causes are almost always the same — wrong encoding, a duplicate key that wasn't there last month, or an invisible character that crept in during a copy-paste. These problems are invisible until it's too late.

The goal was to build a tool that catches those issues before the upload — fast, without sending sensitive business data to any external server, and without requiring a login just to try it. That means all processing happens in the browser using the JavaScript File API and TextDecoder. Your CSV never leaves your device.

What CSV Checker does

CSV Checker is a set of browser-based utilities for anyone who works with CSV files regularly — data analysts, system integrators, operations staff, developers, and anyone who exports data from one system and imports it into another.

Single-file check

Validates one CSV: format, encoding, header, delimiter, empty rows, column count mismatch, and more.

Compare two files

Compares two CSVs and shows added, deleted, and changed rows with colour highlighting.

Encoding fix (garbled text)

This tool converts garbled CSV or text files to UTF-8 (with BOM) entirely in your browser. Files are never uploaded to a server; all processing happens on your device.

Features

Drag & drop: Drop a file on the area to start conversion.
Auto encoding detection: Detects Shift-JIS, EUC-KR, CP949, and others, then decodes the file correctly.
Manual encoding: If auto-detect is wrong, choose the source encoding from the dropdown.
Preview: View the first 10 lines of the converted text to confirm it looks correct.
UTF-8 BOM download: Saves in a format that opens correctly in Excel. The file is named fixed_originalname.

Supports CSV as well as TXT, LOG, and other text-based files. Safe for sensitive or work data since nothing is sent to a server.

Who it's for

CSV Checker is useful whenever you deal with CSV files that need to be reliable:

Before uploading to a database or SaaS system — catch encoding mismatches, duplicate primary keys, and malformed rows before they cause an import error or silently corrupt data.
Monthly data reconciliation — quickly see what changed between last month's export and this month's, without opening two files side by side in a spreadsheet and eyeballing them.
Debugging import failures — when an import "should work" but keeps failing, the format check and single-file check give you a checklist of exactly what's wrong.
Handling files from external sources — supplier exports, government open data, CRM exports — each system uses a different encoding, delimiter, and quoting style. CSV Checker normalises them.
Working with sensitive data — because nothing is uploaded, you can safely use CSV files containing personal information, customer records, or financial data.

Privacy and security philosophy

Most online file tools work by uploading your file to a server, processing it there, and returning a result. That means your data is transmitted over the network, temporarily stored on someone else's infrastructure, and subject to that service's privacy policy and data retention rules.

CSV Checker takes a different approach: all processing happens inside your browser using the JavaScript File API, FileReader, and TextDecoder. The file bytes never leave your device. There is no backend processing step, no temporary storage, and no transmission. You can verify this by running the tool with your browser's network tab open — you will see no outbound request carrying your file data.

This is especially important for files that contain personal data (names, emails, IDs), financial records, or internal business information that should not be shared with third-party services.

How it differs from other CSV tools

No upload required. Most browser-based CSV tools upload your file to their server. CSV Checker processes everything locally — this is not a marketing claim but a technical constraint built into how the tool works.
Focused on quality issues. Many CSV converters handle format conversion but not data quality. CSV Checker specifically looks for the problems that cause import failures: invisible characters, duplicate keys, encoding mismatches, column count drift.
Two-file diff view. Side-by-side CSV comparison with colour-coded added/deleted/changed rows is harder to find in free tools. CSV Checker provides this in the browser with no row limit on the comparison itself.
Multiple tools in one place. Instead of juggling separate tools for encoding conversion, format inspection, duplicate detection, and file comparison, everything is available under one URL.

Technical approach

CSV Checker is a static web application — there is no server-side code that touches your data. The tools are built with vanilla JavaScript and use the following browser APIs:

File API / FileReader — reads file bytes directly from the local filesystem into the browser's memory.
TextDecoder — decodes bytes using the detected or specified character encoding (UTF-8, Shift-JIS, EUC-KR, Windows-1252, etc.).
Blob / URL.createObjectURL — generates downloadable output files entirely in memory, without any server round-trip.

Because processing is CPU-bound on your device, very large files (hundreds of thousands of rows) may be slow. For those cases, the CSV Splitter lets you divide the file into smaller chunks first.

Browser requirements

We recommend the latest Google Chrome or Microsoft Edge for the best experience. These browsers have the most complete support for the File API and TextDecoder features that the tool relies on. Most modern browsers (Firefox, Safari) will also work, but are tested less frequently.

Feedback and contact

CSV Checker is a personal project and is actively maintained. If you find a bug, have a feature request, or a file that the tool handles incorrectly, please reach out at youiny7@gmail.com. Concrete examples (file structure, expected vs. actual behaviour) are the most helpful.