JSON to CSV Converter Online

How Nested JSON Becomes Flat Columns

CSV is a rectangle. Every row has the same number of fields and every field holds a single scalar value. JSON is a tree, so something has to give when you convert one into the other. The standard answer is dot notation: a nested key path is collapsed into one column name by joining the levels with a full stop.

An object like {"id": 7, "address": {"city": "Leeds", "postcode": "LS1 4DY"}} produces three columns: id, address.city, and address.postcode. Nesting can go as deep as you like, and the column name simply grows longer. The rule is reversible, which is what makes a lossless round trip back to JSON possible: split each header on the full stop and rebuild the branches.

Records that do not share the same shape are handled by taking the union of all key paths across the array. A record that lacks a particular path gets an empty cell rather than being skipped, so the rectangle stays intact.

What happens to arrays inside a record

Arrays have two reasonable treatments. The first is joining the elements into one cell with a separator such as a pipe, so ["vip", "beta"] becomes vip|beta. This keeps the column count stable no matter how many elements each record has, and it survives a round trip if you pick a separator that never appears in the data. The second is indexed expansion, where tags[0], tags[1], and tags[2] become their own columns. That preserves element identity but the column count is set by the longest array in the whole file, which produces a lot of empty cells.

Arrays of objects are the hard case. An orders array containing three objects with four keys each turns into twelve columns under indexed expansion. When that happens, it is usually a sign the data wants two CSV files with a shared key rather than one wide file.

Side by Side Example

The same three user records, first as a JSON array and then as the CSV the converter produces with flattening enabled.

Note row three. The name contains a comma, so the whole field is wrapped in double quotes. The empty array becomes an empty trailing field rather than the text [].

RFC 4180 Quoting Rules

CSV has no single official standard, but RFC 4180 is the closest thing to one and it is what every serious parser expects. Three rules matter.

1. A field containing the delimiter, a double quote, or a line break must be enclosed in double quotes.
2. A double quote inside a quoted field is escaped by doubling it. The value She said "hi" is written as "She said ""hi""".
3. Line endings inside quoted fields are legal. A parser that splits the file on newlines before parsing quotes will corrupt this data, which is why naive split based parsers fail on real exports.

Fields that need no quoting may still be quoted. Quoting everything is valid and slightly larger, and some teams prefer it because the output is uniform.

Choosing a Delimiter

If a German or French colleague opens your comma separated file and sees everything crammed into column A, the delimiter is the reason. Their Excel is looking for semicolons. Re-export with a semicolon delimiter rather than asking them to change their regional settings.

Excel Gotchas That Silently Corrupt Data

CSV is plain text with no type information, so Excel guesses. Its guesses are frequently wrong and it does not warn you.

The fix is never to double click the file. Use Data then From Text/CSV, and in the import preview set the type of every identifier column to Text before loading. That instructs Excel to leave the characters alone. If you only need the file read back by a program rather than a human, this problem does not exist at all.

Common Pitfalls and Fixes