Getting Started with k6: Modern Load Testing with JavaScript

k6 has become one of my preferred tools for load testing, particularly when working with development teams who are already comfortable with JavaScript. Unlike JMeter’s XML-based test plans, k6 uses plain JavaScript, making it easier to version control, review in pull requests, and integrate into modern development workflows.

Why k6?

After years of using JMeter and LoadRunner, k6 stands out for several reasons:

Feature	k6	JMeter
Scripting language	JavaScript ES6	XML/GUI
Resource usage	Low (Go-based)	High (Java-based)
CI/CD integration	Native	Requires plugins
Learning curve	Low for JS developers	Moderate
Protocol support	HTTP, WebSocket, gRPC	Extensive

The trade-off is that k6 focuses primarily on HTTP-based protocols, while JMeter supports a wider range including JDBC, JMS, and LDAP. For API and web application testing, k6 is often the better choice.

Installation

k6 is distributed as a single binary with no dependencies.

Linux (Debian/Ubuntu):

sudo gpg -k
sudo gpg --no-default-keyring --keyring /usr/share/keyrings/k6-archive-keyring.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C5AD17C747E3415A3642D57D77C6C491D6AC1D69
echo "deb [signed-by=/usr/share/keyrings/k6-archive-keyring.gpg] https://dl.k6.io/deb stable main" | sudo tee /etc/apt/sources.list.d/k6.list
sudo apt-get update
sudo apt-get install k6

macOS:

brew install k6

Windows:

choco install k6

Verify the installation:

k6 version

Your First k6 Script

Create a file called script.js:

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

export const options = {
  vus: 10,
  duration: '30s',
};

export default function () {
  const res = http.get('https://test-api.k6.io/public/crocodiles/');

  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });

  sleep(1);
}

This script runs 10 virtual users for 30 seconds, each making requests with a 1-second pause between iterations.

Running the Test

Execute the script:

k6 run script.js

The output shows real-time metrics:

running (0m30.0s), 00/10 VUs, 285 complete and 0 interrupted iterations
default ✓ [======================================] 10 VUs  30s

     ✓ status is 200
     ✓ response time < 500ms

     checks.........................: 100.00% ✓ 570  ✗ 0
     data_received..................: 1.2 MB  40 kB/s
     data_sent......................: 45 kB   1.5 kB/s
     http_req_duration..............: avg=89.12ms min=76.23ms max=234.56ms
     http_reqs......................: 285     9.5/s
     iteration_duration.............: avg=1.09s   min=1.07s   max=1.23s
     iterations.....................: 285     9.5/s
     vus............................: 10      min=10 max=10
     vus_max........................: 10      min=10 max=10

Ramping Load Patterns

For more realistic scenarios, use stages to ramp users up and down:

export const options = {
  stages: [
    { duration: '2m', target: 50 },   // Ramp up to 50 users
    { duration: '5m', target: 50 },   // Stay at 50 users
    { duration: '2m', target: 100 },  // Ramp up to 100 users
    { duration: '5m', target: 100 },  // Stay at 100 users
    { duration: '2m', target: 0 },    // Ramp down to 0
  ],
};

Thresholds for Pass/Fail

Define success criteria that cause the test to fail if not met:

export const options = {
  thresholds: {
    http_req_duration: ['p(95)<500'],  // 95% of requests under 500ms
    http_req_failed: ['rate<0.01'],    // Less than 1% failure rate
    checks: ['rate>0.99'],             // 99% of checks must pass
  },
};

This is particularly useful in CI/CD pipelines where you want automated pass/fail decisions.

Handling Authentication

For APIs requiring authentication:

import http from 'k6/http';

const BASE_URL = 'https://api.example.com';

export function setup() {
  const loginRes = http.post(`${BASE_URL}/auth/login`, {
    username: 'testuser',
    password: 'testpass',
  });

  return { token: loginRes.json('access_token') };
}

export default function (data) {
  const params = {
    headers: {
      Authorization: `Bearer ${data.token}`,
    },
  };

  http.get(`${BASE_URL}/api/protected-resource`, params);
}

The setup() function runs once before the test and shares data with all virtual users.

Output to JSON for Analysis

Export results for later analysis:

k6 run --out json=results.json script.js

Or stream metrics to InfluxDB for Grafana dashboards:

k6 run --out influxdb=http://localhost:8086/k6 script.js

When to Choose k6

I recommend k6 when:

• Your team is comfortable with JavaScript
• You’re testing REST APIs or web applications
• CI/CD integration is a priority
• You need lightweight, fast execution

Stick with JMeter when:

• You need to test JDBC, JMS, or other non-HTTP protocols
• You prefer a GUI for test design
• You have existing JMeter expertise and test assets

k6 has become an essential part of my performance testing toolkit, particularly for modern API-first applications where its JavaScript syntax and CI/CD integration provide significant advantages over traditional tools.