k6 Load Test Script Error Fix

In this tutorial, you'll learn about k6 load test script error fix. We cover key concepts, practical examples, and best practices.

Your k6 load test throws SyntaxError or ReferenceError — k6 uses a JavaScript runtime with limitations: no require(), no import, no DOM APIs, and no Node.js built-ins.

The Problem

// WRONG — using Node.js features that k6 doesn't support
const axios = require('axios');
const fs = require('fs');

export default function () {
  const response = axios.get('https://api.example.com/users');
  console.log(response.data);
}

ERRO[0000] GoError: Unexpected token 'export' in script.js at 1:1

k6 uses a Go-based JavaScript runtime (goja) that supports ES6 modules with export default but does NOT support CommonJS (require), Node.js APIs, or browser APIs.

Step-by-Step Fix

1. Use k6's HTTP module

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://api.example.com/users');
  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500,
  });
  sleep(1);
}

2. Configure thresholds

export const options = {
  vus: 50,
  duration: '1m',
  thresholds: {
    http_req_duration: ['p(95)<2000'],
    http_req_failed: ['rate<0.01'],
  },
};

3. Handle JSON responses

export default function () {
  const res = http.get('https://api.example.com/users');
  const users = res.json();

  users.forEach((user) => {
    console.log(`User: ${user.name}`);
  });
}

4. Add ramp-up stages

export const options = {
  stages: [
    { duration: '2m', target: 100 },
    { duration: '5m', target: 100 },
    { duration: '2m', target: 0 },
  ],
};

Expected output:

     ✓ status is 200
     ✓ response time < 500ms

     http_req_duration........: avg=123ms   min=45ms   med=98ms   max=890ms
     http_reqs................: 3000   100/s
     vus......................: 50     min=0  max=50

Prevention Tips

• Use import from k6 modules, never require()
• Use http.get(), http.post() instead of axios/fetch
• Use check() for assertions instead of expect/assert libraries
• Set thresholds in options for pass/fail criteria
• Use stages for gradual ramp-up instead of fixed VUs

Common Mistakes with load test error

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world K6 code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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