Over the past year I've had the pleasure of developing a full-stack application using the Remix framework and it was a bit tricky to get Mocha + Chai tests running.

Option 1: Use ts-node

Install ts-node and then add the following script to package.json:

"test": "env TS_NODE_FILES=true TS_NODE_TRANSPILE_ONLY=true TS_NODE_COMPILER_OPTIONS='{\"module\": \"esnext\" }' mocha 'app/**/*.test.ts'"

Also create .mocharc.json in the project root with the following:

{
  "node-option": ["experimental-specifier-resolution=node", "loader=ts-node/esm"]
}

Option 2: Compile to commonjs separately

This option is a bit more involved but if you have some demarcation between modules, e.g. server and client, then it might be worthwhile to have separate compilation for modules in order to have quick running tests. The idea here is to create a separate pipeline-tsconfig.json which compiles code into commonjs rather than esm (note the included folders and resulting output directory hierarchy):

{
  "compilerOptions": {
    "target": "ES2018",
    "module": "commonjs",
    "allowJs": true,
    "outDir": "build-pipeline",
    "rootDir": "app",
    "strict": true,
    "skipLibCheck": true,
    "esModuleInterop": true,
  },
  "include": ["app/core", "app/db", "app/services", "app/pipeline"]
}

The module (in this example everything under pipeline) can then be tested directly via the following packageJson scrips:

"pipeline:build": "rm -rf ./build-pipeline && tsc -p pipeline-tsconfig.json",
"pipeline:watch": "rm -rf ./build-pipeline && tsc -w -p pipeline-tsconfig.json",
"pipeline:test": "npm run pipeline:build && mocha 'build-pipeline/**/*.test.js'",
"pipeline:test:live": "mocha 'build-pipeline/**/*.test.js'",

Thus, run npm run pipeline:watch and thereafter npm run pipeline:test:live to run continuous tests. npm run pipeline:test can be used to only run tests (in a CI environment for example). Happy testing!