Yale SWE JavaScript Module Jest Example

Relevant Stack

• Node.js (runtime)
• Express (server framework)
• Jest (test framework)

Overview

Some groups have been having trouble getting tests that use code with JavaScript ES6 syntax (ex. import instead of require) working. Ideally we recommend that groups use TypeScript and ts-jest to run tests, but if you are already too deep into JavaScript you may find this useful.

Overview of the Problem

In JavaScript projects that use ES6 syntax (ie. if your package.json has type: "module"), testing libraries may freak out because test files aren't interpreted as modules. In other words, while your src code is happy with import, your tests will not be since they aren't interpreted as modules.

Solution

To solve this problem, we need to run Jest using node with ECMAScript Modules enabled (ie. the flag --experimental-vm-modules). Note that this support is still considered experimental by Node as of version 14.13.1. This is why we recommend you either do not use ES6 syntax in your projects, or you use TypeScript instead (preferred). You can read more about this here.

Demo

This repo contains an example hello world server. You can run it locally like so:

# install packages
yarn

# run server
yarn dev

To run the tests (which use ES6 syntax), you can run the following command:

# run tests with ES6 syntax
yarn test

Note that the command above is an alias for the following (see "scripts" in package.json)

# run Jest tests with experimental vm module support
node --experimental-vm-modules node_modules/jest/bin/jest.js