No results for

Powered byAlgolia

expect.js library is no longer maintained

expect.js library has been deprecated in favor of Chaijs.

Please migrate to k6Chaijs library. The documentation below is retained for historical reasons.

The expect module is a JavaScript library that simplifies specifying expectations about the responses from the target system. The design of the expect library was inspired by ava, Jest and Jasmine. If you already know one of these frameworks, using this library should be very simple.

This library is especially useful for:

  • Functional testing, where many asserts are needed
  • Stress testing, where the System Under Test is failing and the test code needs to stay robust.
  • Load testing, where the failures of the System Under Test need to be robustly collected for analysis

⭐️ Source code available on GitHub. Please request features and report bugs through GitHub issues.


There's nothing to install. This library is hosted on jslib and can be imported in the k6 script directly.

import { describe } from '';

Alternatively, you can use a copy of this file stored locally.

Simple example

Let's get started by writing a test for a hypothetical HTTP API that should return a JSON array of objects.

First, create a mytest.js k6 script file.

import { describe } from '';
import http from 'k6/http';
export default function testSuite() {
describe('Basic API test', (t) => {
const response = http.get('');
t.expect(response.status).as('API status code').toEqual(200);

If you are familiar with k6, this is similar to using the built-in group and check functionality but with different names.

When you run this test with k6 run mytest.js the result should look similar to this:

█ Basic API test
✓ response status is 200.

This basic example is not very exciting because the same result can be achieved with group and check, so let's move on to more interesting examples.

Chain of checks

When writing integration tests and performance test, it's often necessary to execute conditional checks. For example, you may want to inspect the JSON body only when the http response is 200. If it's 500, the body is not relevant and should not be inspected.

It's possible to chain checks using the .and() function, as shown below.

import { describe } from '';
import http from 'k6/http';
export default function testSuite() {
describe('Fetch a list of public crocodiles', (t) => {
const response = http.get('');
.as('response status')
.as('number of crocs')

When you run this test with k6 run mytest.js, the result should look similar to this:

The above script should result in the following being printed after execution:

█ Fetch a list of public crocodiles
✓ response status is 200.
✓ has valid json response
✓ number of crocs is greater than 5

More advanced examples can be found in the examples section

describe(name, function)Entry point for creating tests.
expect(value)expect(value) sets the value to be used in comparison by the next function in the chain
and(value)and(value) is similar to expect(value), but can be used in a chain.
as(alias)as(alias) sets a textual representation of the value passed to expect or and.
toEqual(value)The .toEqual(expectedValue) is similar to ===
toBeGreaterThan(expectedValue)Use to verify that received > expected
toBeGreaterThanOrEqual(expectedValue)Use to verify that received >= expected
toBeLessThan(expectedValue)Use to verify that received < expected
toBeLessThanOrEqual(expectedValue)Use to verify that received <= expected
toBeBetween(from, to)Use to verify that expected value is within range.
toBeTruthy()Use .toBeTruthy when you don't care what a value is and you want to ensure a value is true in a boolean context.
toHaveValidJson()Use to verify that the http response has a valid JSON body.