Embedded JavaScript overview

loadr embeds a JavaScript engine (QuickJS — see ADR-001) so dynamic logic lives next to the declarative YAML. JS is usable three ways:

1. Inline expressions

Anywhere ${...} works, ${js: <expr>} evaluates in the VU's runtime:

headers:
  X-Request-Id: "${js: crypto.uuidv4()}"
params:
  page: "${js: Math.ceil(Math.random() * 10)}"

2. Inline script steps

flow:
  - js: "session.counterAdd('pages_viewed', 1)"
  - js:
      script: |
        const row = session.data('users');
        session.vars.greeting = `hello ${row.username}`;
  - js:
      call: warmCache        # an exported function from the module

3. A module (inline or file)

js:
  file: ./script.js          # or  script: |  (inline source)
  timeout: 10s               # per-call wall-clock limit (default 10s)
  memory_limit_mb: 64        # per-VU heap limit (default 64)

The module is an ES module with k6-compatible imports:

import http from 'k6/http';
import { check, sleep, group } from 'k6';
import { Counter, Trend } from 'k6/metrics';

export function setup() { /* once, before VUs start */ return {...}; }
export default function (data) { /* per iteration when exec/default used */ }
export function teardown(data) { /* once, after the run */ }
export function beforeRequest(req) { /* around every YAML request */ return req; }
export function afterRequest(res) { /* ... */ }

Isolation & limits

Every VU gets its own JS runtime and context — no shared mutable state between VUs (matching k6). Each runtime enforces:

• a heap limit (memory_limit_mb) — exceeding it throws;
• a wall-clock interrupt per call (timeout) — infinite loops are killed;
• no filesystem or network access except through the provided APIs (open() is restricted to the test's directory).

Values flow both ways

• Extracted YAML values appear in JS as session.vars.<name>.
• Values set from JS (session.vars.x = ...) are usable in YAML as ${x}.
• setup()'s return value is passed to every scenario function and is readable in hooks.