Convert JSON to CSV, and back, without uploading it

This conversion is the join between two worlds that do not naturally meet. JSON is what systems talk in: nested, typed, happy with missing fields. CSV is what people work in: flat, untyped, and opened in Excel. Most integration work involves moving between them, and most of the pain is at the edges rather than in the middle.

The edges are worth knowing about because they cause silent damage. A CSV has no types, so a reference number like 00123 loses its leading zeros, a long identifier becomes scientific notation, and a date becomes whatever the reader's locale thinks it is. None of these throw an error. They just produce a file that looks fine and is wrong, and the error surfaces three steps later when something fails to match.

Nesting is the other edge. JSON allows a field to contain a list or another object, and CSV does not, so something has to give: flatten it into columns, join it into one cell, or drop it. Whichever you choose, choose it deliberately, because the default is usually to lose the structure quietly and only notice when the import is missing half its relationships.

The live preview here is the point rather than a nicety. Conversion between formats is exactly the kind of operation where you should look at the output before you trust it, because the failure mode is not a crash, it is a plausible file. Check a row you know the answer for, particularly one with a number that should stay a string.

Running in the browser matters for the obvious reason. The JSON that needs converting is usually an API response or an export, which means it usually contains real records: customers, orders, people. Pasting that into a free online converter sends it to somebody else's server, and there is no version of that which is a good idea.

If this conversion is part of a pipeline rather than a one-off, it should not be happening in a browser tab at all. A repeated manual conversion between two systems is a description of an integration that has not been built yet, and it will keep costing an hour a week until it is.