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The k6 module contains k6-specific functionality for writing load tests. This is the core module that provides essential functions like checks, groups, and sleep.

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Import

import { check, fail, group, sleep, randomSeed } from 'k6';

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Functions

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check()

Runs one or more checks on a value and generates a pass/fail result but does not throw errors or interrupt execution upon failure.
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val
any
required
Value to test.
​
sets
object
required
Tests (checks) to run on the value. Each key is a description, and each value is a function that returns a boolean.
​
tags
object
Extra tags to attach to metrics emitted.
​
return
boolean
Returns true if all checks have succeeded, false otherwise.
import http from 'k6/http';
import { check } from 'k6';

export default function () {
  const res = http.get('https://quickpizza.grafana.com/');
  check(res, {
    'response code was 200': (res) => res.status == 200,
  });
}
Checks are not asserts. A failed assertion throws an error, while a check always returns with a pass or failure. Use thresholds to control failure conditions.

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fail()

Immediately throws an error, aborting the current VU script iteration.
​
err
string
Error message that gets printed to stderr.
import http from 'k6/http';
import { check, fail } from 'k6';

export default function () {
  const res = http.get('https://k6.io');
  if (
    !check(res, {
      'status code MUST be 200': (res) => res.status == 200,
    })
  ) {
    fail('status code was *not* 200');
  }
}
fail() does not abort the test or exit with non-0 status. It only aborts the current iteration. To halt test execution, use test.abort() from k6/execution.

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group()

Runs code inside a group. Groups organize results in a test and tag metrics accordingly.
​
name
string
required
Name of the group.
​
fn
function
required
Group body - code to be executed in the group context.
​
return
any
The return value of the function.
import { group } from 'k6';

export default function () {
  group('visit product listing page', function () {
    // ...
  });
  group('add several products to the shopping cart', function () {
    // ...
  });
  group('visit login page', function () {
    // ...
  });
  group('authenticate', function () {
    // ...
  });
  group('checkout process', function () {
    // ...
  });
}
Avoid using group with async functions or asynchronous code. k6 might apply tags in an unreliable or unintuitive way with promises or await.

​
sleep()

Suspends VU execution for the specified duration.
​
t
number
required
Duration, in seconds.
import { sleep } from 'k6';
import http from 'k6/http';

export default function () {
  http.get('https://k6.io');
  sleep(Math.random() * 30);
  http.get('https://k6.io/features');
}
Don’t use sleep with async code like promises or event handlers. sleep blocks VU execution and prevents the event loop from processing. Use setTimeout from k6/timers for async code instead.

​
randomSeed()

Sets seed to get a reproducible pseudo-random number using Math.random.
​
int
integer
required
The seed value.
import { randomSeed } from 'k6';

export const options = {
  vus: 10,
  duration: '5s',
};

export default function () {
  randomSeed(123456789);
  const rnd = Math.random();
  console.log(rnd);
}
Using the same seed value across iterations produces the same sequence of random numbers.

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