Add typescript, babel typescript, add the plugin in babelrc, and update the Babel cli build command in package.json to look for ts file extensions.
That enabled me to start incrementally adding typescript into an existing node.js codebase, I was surprised how easy it was.
package.json (scripts)
BABEL_ENV=development babel -d build/ src/ --extensions \".ts,.tsx,.js\" --quiet --source-maps inline
"type-check": "tsc",
@babel/preset-typescript, typescript
presets: ["@babel/typescript"]
{
"compilerOptions": {
"target": "esnext",
"module": "commonjs",
"declaration": true,
"outDir": "build",
"strict": true,
"allowSyntheticDefaultImports": true,
"esModuleInterop": true,
"skipLibCheck": true
}
}