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NDJSON Filter CLI

A Node CLI that reads newline-delimited JSON from stdin, keeps records matching a field selector, and writes NDJSON to stdout.

node --version

cat events.ndjson | node ndjson-filter.mjs level=error

node --test ndjson.test.mjs

Writes only the records whose selected field equals the given value, one JSON object per line, preserving input order.

What it does

Streams stdin line by line, parses each line as JSON, compares a dotted field path to the expected value, and writes matching records as NDJSON. A malformed line is reported to stderr and skipped so the stream continues.

import { createInterface } from 'node:readline';

const spec = process.argv[2];
if (!spec || !spec.includes('=')) { console.error('usage: ndjson-filter <field>=<value>'); process.exit(1); }
const [field, want] = spec.split('=', 2);
const value = want === 'true' ? true : want === 'false' ? false : want;

const get = (obj, path) => path.split('.').reduce((o, k) => (o == null ? o : o[k]), obj);

const rl = createInterface({ input: process.stdin, crlfDelay: Infinity });
for await (const line of rl) {
  if (!line.trim()) continue;
  try {
    const rec = JSON.parse(line);
    if (get(rec, field) === value) process.stdout.write(JSON.stringify(rec) + '\n');
  } catch {
    process.stderr.write('skipping malformed line\n');
  }
}

Run and verify

cat events.ndjson | node ndjson-filter.mjs level=error

Run the test command to check the implementation: node --test ndjson.test.mjs.

Expected Output

Given three events of which one has level “error”, the filtered stream contains only that line:

{"id":7,"level":"error","msg":"boom"}

Limitations

Equality only: no ranges or regular expressions. The field path supports dots but not array indexing, so values inside arrays must be flattened first.