NodeJS/Javascript Testing Using the Test Runner Module

The test runner API is a new set of built-in tools allowing you to natively create tests for your code. No more need for Jest, Mocha, or Ava! This will bring Node up with most modern languages (Dart, Rust, GO) and reduce the package size of your code base.

The test runner API is not a drop-in replacement for other frameworks, so you must migrate all of your tests to the new syntax. However, its syntax is very straightforward and takes inspiration from the existing frameworks. This will make transitioning to the built-in test runner much easier.

Getting Started

Installing Node v19.0.0

To get the latest version of Node, you can either install version v19.0.0 from the Node.JS website here, or you can install nvm. NVM is a node version manager, allowing you to run multiple versions of Node on your computer.

Installing NVM

1curl -o- | bash

To install Node v19.0.0

1nvm install 19
2node -v

To install a specific version use the nvm install command if you want to switch to a certain version of Node that you have previously installed

Importing the Test Runner

The test runner module lives inside node:test, below is the basic syntax for importing it into your script.

1import test from "node:test";

To run the tests, use the --test flag on Node and specify the folder where the tests are located at.

1node --test src/*.mjs


Test is the main method that will be used to define the tests and it can be used synchronously or asynchronously. The examples below will be using the assert module which is also a built-in method.

 1import test from "node:test";
 2import assert from "node:assert/strict";
 4test("synchronous passing test", (t) => {
 5  // This test passes because it does not throw an exception.
 6  assert.equal(1, 1, "Sync Test Passed 1 == 1");
 9test("asynchronous passing test", async (t) => {
10  // This test passes because the Promise returned by the async
11  // function is not rejected.
12  assert.equal(1, 1, "Async Test Passed 1 == 1");

To run this test, save the code above as a mjs file and run the following command.

1node --test ./*.mjs


 1✔ synchronous passing test (0.72425ms)
 2✔ asynchronous passing test (0.050875ms)
 3ℹ tests 2
 4ℹ suites 0
 5ℹ pass 2
 6ℹ fail 0
 7ℹ cancelled 0
 8ℹ skipped 0
 9ℹ todo 0
10ℹ duration_ms 54.186084


Now that we have run a test, let's dive into how the assert method works. Assert is a method that enables a deeper level of comparison than the traditional if statements allow. Assert also throw AssertionError's notifying you of potential issues within your tests.

We won't go over every method of the assert module here but I feel it's important to go over a few so you can have some background knowledge for the rest of this tutorial. deepEqual enables deep comparison of objects and their nested values. Below is a comparison of two multi-dimensional arrays with one value changed to a string '3' this will throw an assertion error

1assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, "3"]], 4, 5]);


 1AssertionError: Expected values to be strictly deep-equal:
 2  + actual - expected
 4    [
 5      [
 6        [
 7          1,
 8          2,
 9  +       3
10  -       '3'
11        ]
13      4,
14      5
15    ]

To learn more about the other methods provided by assert check out the official documentation here.


Describe/It is an alternative syntax to the regular test() syntax. Describe defines the suite of tests and It defines the test inside.


1import { describe, it } from "node:test";


 1describe("A thing", () => {
 2  it("should work", () => {
 3    assert.strictEqual(1, 1);
 4  });
 6  it("should be ok", () => {
 7    assert.strictEqual(2, 2);
 8  });
10  describe("a nested thing", () => {
11    it("should work", () => {
12      assert.strictEqual(3, 3);
13    });
14  });

By declaring a suite of tests, the output when is organized into groups based on the nested level when defined. Below is what you should expect when running the code above.

 1▶ A thing
 2  ✔ should work (0.19025ms)
 3  ✔ should be ok (0.04475ms)
 4  ▶ a nested thing
 5    ✔ should work (0.106542ms)
 6  ▶ a nested thing (0.1605ms)
 8▶ A thing (1.167875ms)
10ℹ tests 3
11ℹ suites 2
12ℹ pass 3
13ℹ fail 0
14ℹ cancelled 0
15ℹ skipped 0
16ℹ todo 0
17ℹ duration_ms 44.25425

Node test runner is an exciting new feature that will change how tests are made in nodejs. I find that the syntax is very easy to learn and I am excited to use this feature in new projects.