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NDJSON solves the problem of processing large JSON datasets without loading everything into memory. Instead of wrapping a million objects in a JSON array (which requires parsing the entire file), NDJSON puts one complete JSON object on each line. A processor reads one line, parses one object, processes it, and moves on. Memory usage stays constant regardless of file size.
The format is everywhere in backend systems: application logs, database exports, event streams, ETL pipelines, and data processing workflows. Docker logs are NDJSON. Many API bulk endpoints return NDJSON streams. Elasticsearch's bulk API expects NDJSON input.
The format has multiple names — NDJSON, JSON Lines (.jsonl), and line-delimited JSON are all the same thing: one valid JSON value per line, separated by newlines. The simplicity is the feature. Any language that can read a file line by line and parse JSON can process NDJSON, no special library needed.