JSONL Viewer & Formatter Handles large files smoothly: view, search, validate and convert

Why use a dedicated JSONL viewer?

JSONL (JSON Lines), also called NDJSON, stores one JSON value per line. It is widely used for LLM fine-tuning datasets, batch API inputs and outputs, logs and data exports. A JSONL file is not one valid JSON document, so ordinary JSON formatters throw errors on it, and multi-megabyte files often freeze browser-based editors.

How this tool stays fast

• Files are read in chunks and only a line index is built; the whole file is never turned into one giant string.
• The list uses virtual scrolling, so only the few dozen visible lines are rendered, even with millions of lines.
• Only the line you click is parsed and syntax-highlighted.
• Validation, search and export run in batches with progress and can be cancelled at any time.

Common uses

• Check that every line of a fine-tuning dataset is valid JSON and find the broken line numbers.
• Search a large conversation log by keyword and export the matches as a new JSONL file.
• See how often each top-level field appears to spot inconsistent records.
• Convert JSONL to a standard JSON array, or export a minified JSONL.

When exporting to a JSON array, invalid lines are skipped and the count is shown. Very large files (hundreds of MB) need extra memory to export; if the browser slows down, filter with search first and export only what you need. See Line-delimited JSON on Wikipedia for the format.