ES6 modules for ES5 output

[pkozlowski-opensource]

As of today specifying ES6 as a module format to transpile to with ES5 as a target will result in the "error TS1204: Cannot compile modules into 'es2015' when targeting 'ES5' or lower." error.

But having ES6 modules in the ES5 output would be beneficial for ES6-based tree-shakers like Rollup.

Currently one can ES5 output with ES6 modules by using ts.transpileModule(...) but the output is broken in some cases. For example transpiling:

ts.version // 1.7.4
ts.transpileModule('export class A {}', {
  compilerOptions: {
    target: ts.ScriptTarget.ES5,
    module: ts.ModuleKind.ES6
  }
})

yields:

var A = (function () {
    function A() {
    }
    return A;
})();
A = A;

Notice: A = A; which should probably be export { A };

In-depth discussion and more examples in rollup/rollup-plugin-typescript#9

Given the above I've got a question: do you plan to support ES6 modules for ES5 output? If not are you planning to provide a TS-specific tree-shaking bundler?

[pkozlowski-opensource]

Oh, it looks like @Victorystick opened a similar #5790, closing this one as a duplicate and moving the discussion in there.