NDJSON / JSON Lines: one JSON object per line

NDJSON (newline-delimited JSON) stores many JSON objects as separate lines. It’s popular for logs, data pipelines, and streaming.

What it looks like

Each line is valid JSON by itself:

{"ts":"2026-03-19T10:00:00Z","level":"info","msg":"started"}
{"ts":"2026-03-19T10:00:01Z","level":"warn","msg":"retrying","attempt":2}
{"ts":"2026-03-19T10:00:02Z","level":"info","msg":"done"}

How it differs from a JSON array

A JSON array is one JSON document:

[
  {"id": 1},
  {"id": 2}
]

NDJSON is many JSON documents separated by newlines.

Why it’s useful

• Streaming: process line-by-line without loading the whole file
• Appending: easy to append new records
• Robust logs: a bad line doesn’t necessarily break the whole file

Common mistakes

• Trailing commas: not allowed in JSON
• Pretty printing the whole file: can break “one object per line” conventions
• Mixing formats: don’t mix NDJSON and JSON arrays unless the consumer expects it

Related: Large JSON tips · Validate JSON