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Unit Testing Framework

Status: Accepted
- Supersedes ES-compatible unit test frameworks (ADR#0000)
Deciders: Monitor team
Date: 2023-07-25

Technical Story: MNTOR-1823

Context and Problem Statement

With the transition to Next.js/React, the team wants to consider testing frameworks again, to confirm what works best for the new architecture.

Decision Drivers

What works best for the current framework, Next.js
Which provides the best experience for developers

Considered Options

No Change - Ava (ES6+ compatible test runner)
Jest (with the Rust Compiler)
Jest (with Babel)

Decision Outcome

Jest (with the Rust Compiler)

This decision was made in an effort to go with what has the most support for our framework (Nextjs).

Positive Consequences

Easy to port past tests (Already completed in #3168)
Compability with Nextjs framework

Negative Consequences

Still doesn't fully support ES6+ modules

Pros and Cons of the Options

No Change - Ava (ES6+ compatible test runner)

Good, because the system under test and the test code are pure ES modules
Good, because the system under test needs no changes
Bad, because the unit tests need to be rewritten to use the new non-jest test runner and mocking library

Jest (with the Rust Compiler)

Use Jest and the default compiler, SWC

Good, Next.js now has built-in configuration for Jest
Good, Auto mocks of .css, .module.css (and their .scss variants), and image imports
Good, Automatically sets up transform using SWC
Good, Loads .env (and all variants) into process.env
Good, Ignores node_modules from test resolving and transforms
Good, Ignores .next from test resolving
Good, Loads next.config.js for flags that enable experimental SWC transforms
Good, SWC is faster than Babel
Good, needs no additional configuration to be able to test UI code
Bad, does not fully support ES6+ modules

Jest (with Babel)

Use Jest and Babel

Good, Next.js is configured
Bad, slower than SWC
Bad, stylesheets and Image imports need to be mocked

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