property workerId | Node.js node:test module | Bun

The unique identifier of the worker running the current test file. This value is derived from the NODE_TEST_WORKER_ID environment variable. When running tests with --test-isolation=process (the default), each test file runs in a separate child process and is assigned a worker ID from 1 to N, where N is the number of concurrent workers. When running with --test-isolation=none, all tests run in the same process and the worker ID is always 1. This value is undefined when not running in a test context.

This property is useful for splitting resources (like database connections or server ports) across concurrent test files:

import { test } from 'node:test';
import { process } from 'node:process';

test('database operations', async (t) => {
// Worker ID is available via context
console.log(`Running in worker ${t.workerId}`);

// Or via environment variable (available at import time)
const workerId = process.env.NODE_TEST_WORKER_ID;
// Use workerId to allocate separate resources per worker
});