Mocha + TypeScript + ESM

.mocharc.json

{
    "extension": ["ts"],
    "node-option": ["experimental-specifier-resolution=node", "loader=ts-node/esm"],
    "spec": ["test/**/*.test.ts"]
}

package.json

{
    "type": "module",
    "sideEffects": false,
    "scripts": {
        "test": "mocha"
    },
    "devDependencies": {
        "@types/chai-as-promised": "^7.1.4",
        "@types/mocha": "^9.0.0",
        "@types/node": "^16.9.1",
        "chai": "^4.3.4",
        "chai-as-promised": "^7.1.1",
        "mocha": "^9.1.3",
        "ts-node": "^10.3.0",
        "tslib": "^2.3.0",
        "typescript": "^4.4.3"
    }
}

tsconfig.json

{
    "compilerOptions": {
        "outDir": "lib",
        "noEmit": false,
        "declaration": true,
        "sourceMap": true,
        "noEmitOnError": true,
        "stripInternal": true,
        "target": "es6",
        "module": "esnext",
        "moduleResolution": "node",
        "strict": true,
        "esModuleInterop": true,
        "resolveJsonModule": true,
        "isolatedModules": true,
        "typeRoots": ["node_modules/@types", "types"]
    },
    "include": ["src", "types", "test"],
    "exclude": ["node_modules"]
}

thank you! one note: you can add "no-warnings" to line 3 to avoid the nasty message message on each test run.

Any way to also add path aliases resolution? (resolving paths defined within tsconfig.json>paths)

Watch mode is broken with this configuration but adding --parallel seems to fix the problem.

mochajs/mocha#4374 (comment)