JSONL Converter

Worked Example

Input JSONL:

{"id":1,"event":"signup","user":"alice"}
{"id":2,"event":"purchase","user":"bob","amount":29.99}

Output JSON array:

[
  {
    "id": 1,
    "event": "signup",
    "user": "alice"
  },
  {
    "id": 2,
    "event": "purchase",
    "user": "bob",
    "amount": 29.99
  }
]

Numbers Keep Their Digits

The tool uses JSON.parse() only to check the syntax. It writes every number with the characters from the input, so IDs above 2⁵³, trailing zeros and exponents survive both directions:

{"order_id":9007199254740993,"total":1.50,"rate":1e-7}
{"order_id":9007199254740994, "note":"caf\u00e9", "total":0}

[
  {
    "order_id": 9007199254740993,
    "total": 1.50,
    "rate": 1e-7
  },
  {
    "order_id": 9007199254740994,
    "note": "café",
    "total": 0
  }
]

A plain JSON.parse() followed by JSON.stringify() would write 9007199254740992 and 1.5. Strings are a different case: the tool writes them in the standard JSON.stringify() form, so "caf\u00e9" becomes “café”, and the space after a comma disappears in compact JSONL.

Copy and Download

The Copy buttons and both downloads use the result of the last successful conversion. After JSONL → JSON, Download .json saves the array shown on the right, and the JSONL Copy and Download .jsonl give the converted records one per line in compact form. That is not the text you pasted: invalid lines left out by Convert valid lines only are gone and spacing is removed. After JSON → JSONL, Download .json saves your array formatted with the current Pretty JSON output setting.

The buttons are disabled before the first conversion and again as soon as you edit either panel, change Ignore empty lines or Convert valid lines only, click Clear (or press Ctrl+L / Cmd+L in the tool), or a conversion or a file read fails. Changing Pretty JSON output runs JSONL → JSON again when the JSONL panel has text. A failed conversion also empties the output panel, so you cannot copy a result that belongs to older input. Automatic validation on its own never enables them.

JSONL vs JSON Arrays

A JSON array wraps every record inside one large document. JSONL writes each record as a standalone JSON value on its own line. That line-oriented shape works well for logs, streaming exports, command-line processing, and large datasets where each record can be handled independently.

Example: One Bad Line in a Three-Line Export

Paste this into the JSONL panel and wait 350 ms for automatic validation:

{"id":1,"event":"signup"}
{"id":2,"event":"purchase",}
{"id":3,"event":"cancel"}

The counters show 3 lines, 2 valid, 1 error, 0 empty. The issue panel lists Line 2 with the browser’s parser message; Chrome reports Expected double-quoted property name in JSON at position 27 (line 1 column 28) (the position counts from the start of that line). Firefox and Safari word the message differently. JSONL → JSON stops with an error until you fix the line or tick Convert valid lines only. With the option on, the tool outputs records 1 and 3, reports “Converted 2 JSONL records to a JSON array.”, and Download .jsonl saves:

{"id":1,"event":"signup"}
{"id":2,"event":"purchase",}
{"id":3,"event":"cancel"}

{"id":1,"event":"signup"}
{"id":3,"event":"cancel"}

Command-Line Equivalents

For repeatable scripts, jq does the same conversions (jq 1.7.1 output):

jq -c '.[]' data.json > data.jsonl   # JSON array → JSONL
jq -s '.' data.jsonl > data.json     # JSONL → JSON array
jq -c . bad.jsonl                    # stops at the first bad line:
# jq: parse error: Expected another key-value pair at line 2, column 28

jq stops at the first error. The tool parses every line and lists all failing lines at once.

Limits

• Duplicate keys keep the last value, as JSON.parse() does: {"x":1,"x":2} becomes {"x":2}, at the place of the first x.
• Keys that look like array indexes move to the front of their object, in ascending order, as in a JavaScript object: {"b":1,"2":2,"1":3} becomes {"1":3,"2":2,"b":1}. Values do not change.
• JSON → JSONL accepts only a top-level array. An object or a single value produces “JSON input must be an array.”
• Each record must sit on one line. A pretty-printed object spread across several lines fails on every line it spans. Lines are split on line feeds and trimmed, so Windows CRLF files work.
• The issue panel shows the first 40 failing lines; the error counter still counts all of them.