Overriding `tsconfig.json` for ts-node in mocha

Question

Is it possible to override which tsconfig.json ts-node uses when called from mocha?

My main tsconfig.json contains "module": "es2015", but I want to use "module": "commonjs" for ts-node only.

I tried this

mocha --compilers ts:ts-node/register,tsx:ts-node/register \
    --compilerOptions '{"module":"commonjs"}' \
    --require ts-node/register test/**/*.spec.ts*

but it did not work:

SyntaxError: Unexpected token import
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Module.m._compile (/usr/lib/node_modules/ts-node/src/index.ts:406:23)
    at Module._extensions..js (module.js:422:10)
    at Object.require.extensions.(anonymous function) [as .tsx] (/usr/lib/node_modules/ts-node/src/index.ts:409:12)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:16:19)
    at /usr/lib/node_modules/mocha/lib/mocha.js:222:27
    at Array.forEach (native)
    at Mocha.loadFiles (/usr/lib/node_modules/mocha/lib/mocha.js:219:14)
    at Mocha.run (/usr/lib/node_modules/mocha/lib/mocha.js:487:10)
    at Object.<anonymous> (/usr/lib/node_modules/mocha/bin/_mocha:458:18)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:146:18)
    at node.js:404:3

Answer 1

You need to set the configuration through the TS_NODE_COMPILER_OPTIONS environment variable

Example code on an unix machine:

TS_NODE_COMPILER_OPTIONS='{"module":"commonjs"}' \
mocha --require ts-node/register 'test/**/*.spec.{ts,tsx}'

---

Explanation extracted from the repository documentation

---

CLI and Programmatic Options

Environment variable denoted in parentheses.

• -T, --transpile-only Use TypeScript's faster transpileModule (TS_NODE_TRANSPILE_ONLY, default: false)
• -I, --ignore [pattern] Override the path patterns to skip compilation (TS_NODE_IGNORE, default: /node_modules/)
• -P, --project [path] Path to TypeScript JSON project file (TS_NODE_PROJECT)
• -C, --compiler [name] Specify a custom TypeScript compiler (TS_NODE_COMPILER, default: typescript)
• -D, --ignore-diagnostics [code] Ignore TypeScript warnings by diagnostic code (TS_NODE_IGNORE_DIAGNOSTICS)
• -O, --compiler-options [opts] JSON object to merge with compiler options (TS_NODE_COMPILER_OPTIONS)
• --files Load files from tsconfig.json on startup (TS_NODE_FILES, default: false)
• --pretty Use pretty diagnostic formatter (TS_NODE_PRETTY, default: false)
• --skip-project Skip project config resolution and loading (TS_NODE_SKIP_PROJECT, default: false)
• --skip-ignore Skip ignore checks (TS_NODE_SKIP_IGNORE, default: false)
• --log-error Logs errors of types instead of exit the process (TS_NODE_LOG_ERROR, default: false)
• --prefer-ts-exts Re-order file extensions so that TypeScript imports are preferred (TS_NODE_PREFER_TS_EXTS, default: false)