application/x-ndjson — Newline-Delimited JSON for Streaming

What is NDJSON?

NDJSON (Newline-Delimited JSON), also known as JSON Lines, is a format where each line contains a valid JSON value, separated by newline characters (\n).

MIME Type

application/x-ndjson

Common file extensions: .ndjson, .jsonl

Example

{"id": 1, "name": "Alice", "role": "admin"}
{"id": 2, "name": "Bob", "role": "user"}
{"id": 3, "name": "Carol", "role": "user"}

Why Not Standard JSON?

Standard application/json requires the entire document to be a single valid JSON structure (typically an array). This means:

• You must wait for the complete response before parsing
• Large datasets require buffering the entire body in memory

NDJSON solves both problems by allowing line-by-line streaming:

• Parse each line independently as it arrives
• No need to buffer the entire payload
• Process millions of records with constant memory

Streaming API Example

GET /api/events/stream HTTP/1.1
Accept: application/x-ndjson

HTTP/1.1 200 OK
Content-Type: application/x-ndjson
Transfer-Encoding: chunked

{"event": "user.created", "timestamp": 1700000000}
{"event": "order.placed", "timestamp": 1700000001}

Common Use Cases

• Elasticsearch Bulk API — uses NDJSON for bulk indexing
• Log aggregation — structured logs in NDJSON format
• Data pipelines — streaming data between microservices
• LLM APIs — streaming token-by-token responses