k6 — Modern Load Testing Tool Using Go and JavaScript

What it is

k6 is a modern load testing tool built by Grafana Labs. You write test scripts in JavaScript (ES6), and k6 executes them in a high-performance Go runtime (not Node.js). It is CLI-first, developer-friendly, and built for CI/CD integration.

k6 targets developers and QA engineers who need to validate API performance, test for regressions, and simulate concurrent user load. It works locally, in CI pipelines, and with Grafana Cloud for distributed testing and result analysis.

How it saves time or tokens

k6 combines the familiarity of JavaScript with the performance of Go. Unlike JMeter or Gatling, there is no GUI to configure. Tests are code, which means they are version-controlled, reviewed in PRs, and run in CI/CD automatically. Built-in thresholds fail the test if performance SLOs are not met, catching regressions before deployment. The CLI-first workflow means load tests run alongside unit tests in the same pipeline.

How to use

1. Install k6:

brew install k6              # macOS
sudo apt install k6          # Debian/Ubuntu

1. Write a load test script:

import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    { duration: '30s', target: 50 },
    { duration: '1m',  target: 50 },
    { duration: '10s', target: 0 },
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'],
    http_req_failed: ['rate<0.01'],
  },
};

export default function () {
  const res = http.get('https://api.example.com/health');
  check(res, {
    'status is 200': (r) => r.status === 200,
  });
  sleep(1);
}

1. Run the test:

k6 run load-test.js
k6 run --vus 100 --duration 2m load-test.js

Example

A CI/CD-ready load test with multiple scenarios:

import http from 'k6/http';
import { check } from 'k6';

export const options = {
  scenarios: {
    smoke: {
      executor: 'constant-vus',
      vus: 1,
      duration: '10s',
    },
    load: {
      executor: 'ramping-vus',
      startVUs: 0,
      stages: [
        { duration: '1m', target: 100 },
        { duration: '3m', target: 100 },
        { duration: '1m', target: 0 },
      ],
      startTime: '15s',
    },
  },
  thresholds: {
    http_req_duration: ['p(99)<1000'],
    http_req_failed: ['rate<0.005'],
  },
};

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Common pitfalls

• Running k6 from a single machine limits the maximum number of virtual users. For large-scale tests, use k6 Cloud or distribute across multiple machines.
• Not setting thresholds means tests always pass regardless of performance. Define SLO-based thresholds for every test.
• Testing against production without rate limiting can cause outages. Use staging environments or set conservative VU limits for production tests.