Allow built ESM to be native ES2015+ (not ES5)

[justin-schroeder]

Current Behavior

When creating a new library that needs no legacy support (ES6+ is just 👌) — I seem unable to produce esm outputs that are not transpiled to ES5. Reproduction:

1. Create a new project and add

   npx tsdx create my-es6-module

2. Add target: 'ES2015 to compileOptions in tsconfig.js:

   {
   // see https://www.typescriptlang.org/tsconfig to better understand tsconfigs
   "include": ["src", "types"],
   "compilerOptions": {
   "module": "esnext",
   "target": "ES2015",
   "lib": ["dom", "esnext"],
   "importHelpers": true,
   // output .d.ts declaration files for consumers
   "declaration": true,
   // output .js.map sourcemap files for consumers
   "sourceMap": true,
   // match output dir to input dir. e.g. dist/index instead of dist/src/index
   "rootDir": "./src",
   // stricter type-checking for stronger correctness. Recommended by TS
   "strict": true,
   // linter checks for common issues
   "noImplicitReturns": true,
   "noFallthroughCasesInSwitch": true,
   // noUnused* overlap with @typescript-eslint/no-unused-vars, can disable if duplicative
   "noUnusedLocals": true,
   "noUnusedParameters": true,
   // use Node's module resolution algorithm, instead of the legacy TS one
   "moduleResolution": "node",
   // transpile JSX to React.createElement
   "jsx": "react",
   // interop between ESM and CJS modules. Recommended by TS
   "esModuleInterop": true,
   // significant perf increase by skipping checking .d.ts files, particularly those in node_modules. Recommended by TS
   "skipLibCheck": true,
   // error out if import and file system have a casing mismatch. Recommended by TS
   "forceConsistentCasingInFileNames": true,
   // `tsdx build` ignores this option, but it is commonly used when type-checking separately with `tsc`
   "noEmit": true,
   }
   }

3. Some sample code in the index.ts:

export function logSecond(...args: any[]) {
  if (args.length > 1) {
    const [,second] = args;
    console.log(second);
  }
}

3. Run yarn build

4. Check the dist/es-6-module.esm.js output:

   
   function logSecond() {
   for (var _len = arguments.length, args = new Array(_len), _key = 0; _key < _len; _key++) {
   args[_key] = arguments[_key];
   }
   
   if (args.length > 1) {
   var second = args[1];
   console.log(second);
   }
   }

export { logSecond };

### Desired Behavior

Ideally there would be a way to not build for ES5. 

### Suggested Solution

I'm wondering if this has more to do with Rollup or Babel because modern TS doesn't do this? I've messed around with a `tsdx.config.js`, but still with no improvements.

### Who does this impact? Who is this for?

Library authors wanting to drop legacy node and browser support.

[justin-schroeder]

FWIW — TSDX is an awesome project, was really helpful in bootstrapping a new typescript project. It's helpful to have an opinionated approach like this. The project I'm working on is trying to shave every byte out it can so this issue was a bit of a showstopper for me — in the meantime i've moved to straight tsc compilation with a custom terser implementation to shave a few extra bytes off.

[xdoer]

You can create a babel config file, and specify true to targets.esmodules option.

[remcohaszing]

I found out this can be done by adding the following .browserslistrc file in your project root:

last 2 Chrome versions
last 2 Firefox versions
last 2 Safari versions
node 12

It can’t be too new, because tsdx uses an older version of terser which doesn’t understand some syntax (such as optional?.chaining)