Proposal: `export as namespace` for UMD module output

Current problem

TypeScript supports UMD output but does not support exporting as a global namespace.

Syntax

  NamespaceExportDeclaration:
   export as namespace IdentifierPath

Behavior

export var x = 0;
export function y() {}
export default {};
export as namespace My.Custom.Namespace;

// emits:

(function (global, factory) {
  if (typeof module === "object" && typeof module.exports === "object") {
    var v = factory(require, exports);
    if (v !== undefined) module.exports = v;
  }
  else if (typeof define === "function" && define.amd) {
    define(["require", "exports"], factory);
  }
  else {
    global.My = global.My || {};
    global.My.Custom = global.My.Custom || {};
    global.My.Custom.Namespace = global.My.Custom.Namespace || {};
    var exports = global.My.Custom.Namespace;
    factory(global.require, exports);
  }
})(this, function (require, exports) {
  "use strict";
  exports.__esModule = true;
  exports.x = 0;
  function y() { }
  exports.y = y;
  exports["default"] = {};
});

Note

• This proposal basically follows Babel behavior.
• Importing any module without a module loader will throw in this proposal. A further extension may use global namespaces as Babel does.
• Babel overwrites on the existing namespace whereas this proposal extends the existing one, as TS namespace does.
• Rollup has a special behavior where export default X works like CommonJS module.exports = X whereas this proposal does not.

Prior arts: Babel exactGlobals, Webpack multi part library, Rollup output.name option with namespace support

See also

#8436
#10907
#20990

The issue names no source files, tests, or entry points. Begin by comparing the proposed NamespaceExportDeclaration and UMD behavior with related issues #8436, #10907, and #20990; done requires an agreed design and implementation criteria for global namespace output.