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

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)