CSV to JSON

Typing values is where these converters differ

Every cell in a CSV is text. JSON has types. Deciding which text becomes a number is the entire interesting part of the conversion, and it is where a converter can quietly ruin your data.

• A leading zero means it is not a number. 00012345 is an invoice number, a SKU or a zip code. Written as 12345 it no longer matches anything upstream. Here it stays a string: a value is only converted to a number when writing that number back out gives the original text character for character.
• A thousands separator is not a number either. 8,600 is ambiguous — a separator in English, a decimal comma in German. Rather than guess, it stays a string, so you can see it and fix the source. What is unambiguous (8600, -12.5, 1.2e3) is typed.
• Empty is null, not an empty string. A blank cell becomes null so that "no value" survives the round trip instead of turning into "", which most consumers treat as a value.

If you want none of that, turn off Type numbers and booleans and every value comes out as a string.

Three output shapes, for three different consumers

• One object per row is what an API or a JavaScript program usually wants: [{"invoice_no":"00012345","amount":8600}, …].
• Array of arrays keeps the header as the first row. It is compact, it preserves column order exactly, and it is what charting libraries and Google Sheets' API take.
• One array per column — {"invoice_no":[…],"amount":[…]} — is the shape dataframe libraries load fastest, and the one to pick when the columns are long and few.

Duplicate and empty headers

Two columns called amount would silently overwrite each other in an object, so the second becomes amount_2. A column with no header gets its spreadsheet letter (A, B, C) rather than an empty key.

Encodings, so the JSON is not mojibake

A CSV exported from a Japanese or Taiwanese system is often Shift-JIS or Big5, not UTF-8. Dropping the file here detects the encoding rather than assuming UTF-8, and everything written out is UTF-8 — which is the only encoding JSON is defined for.