Nothing is uploaded. When you open a file the CSV Editor reads it locally with the browser’s File API, parses it into an in-memory table, and every edit, sort, filter and export happens on your device in JavaScript. There is no server, API, database or tracking in the processing path, so a spreadsheet of customers, finances or any other sensitive data stays on your machine. The tool works offline once cached, and closing the tab discards everything. The one case where data touches the network is if you paste a remote URL for the tool to fetch — and even then the response is downloaded straight into your browser and never sent anywhere else. The only optional network use anywhere on the site is consent-gated advertising, which never sees your file.

The CSV Editor handles Excel both ways. When you upload an .xlsx or .xlsm file it uses a bundled spreadsheet engine to read the first worksheet, flattening rich text and dates into plain cell values, and loads it into the editable grid alongside plain .csv, .tsv and .txt files. When you export, “Excel” produces a genuine .xlsx workbook — a bold header row, auto-sized columns, and values that look like numbers written as real numbers so Excel treats them numerically. You can also export CSV (UTF-8 with a BOM and CRLF line endings, which makes Excel open accented characters correctly) or JSON (an array of objects keyed by your column headers). One caveat: it edits a single flat sheet, so multiple tabs, formulas, cell styling and charts are not preserved — it is a fast data editor, not a full spreadsheet application.

CSV looks simple but breaks the moment a value contains the delimiter, a quote or a line break, so this editor follows RFC 4180 rather than splitting naively on commas. A field wrapped in double-quotes may contain the delimiter, actual line breaks, and literal double-quotes written as two quotes in a row (“”) — the parser tracks quote state to read all of these, and it strips a leading UTF-8 BOM if present. It also auto-detects the delimiter by sampling the start of the file and scoring commas, tabs, semicolons and pipes, which matters for European exports that use semicolons or for TSV files. On the way out it re-quotes any field that contains the delimiter, a quote or a newline and doubles internal quotes, so a value you typed with a comma in it round-trips safely instead of splitting into two columns.

The table is virtualized, meaning it only renders the handful of rows visible in the scroll viewport plus a small buffer, so a file with 50,000 or more rows scrolls, sorts and filters without freezing the page. Editing, undo / redo and multi-cell paste all operate on the in-memory data, so they stay fast at scale. Because everything is held in your browser’s memory, there is a practical ceiling: the tool accepts files up to 50 MB, and extremely large or wide files will use more RAM and can slow down on low-memory devices. For most exports, contact lists, product feeds and analytics dumps this is comfortable headroom; for multi-gigabyte data warehouse extracts a database or command-line tool is the better fit.