babel-plugin-async-to-generator should add #__PURE__ comment

[winstonewert]

Request: add pure comments to async function helpers to aid in dead code elimination.

Input Code

https://babeljs.io/repl/#?babili=false&browsers=&build=&builtIns=false&code_lz=IYZwngdgxgBAZgV2gFwJYHsLwKbKgCwAVgBzbAEWGWAAoAHU7AShgG8AoGGAJ1wW6ysAvuyFA&debug=false&circleciRepo=&evaluate=false&lineWrap=true&presets=es2015%2Creact%2Cstage-2&targets=&version=6.26.0

async function fetchPageData(page) {
  return {}
}

Expected Behavior

Generated code should be able to be removed as dead by uglifyjs/similiar if the async function is not called.

Current Behavior

var fetchPageData = function () {
  var _ref = _asyncToGenerator( /*#__PURE__*/regeneratorRuntime.mark(function _callee(page) {
    return regeneratorRuntime.wrap(function _callee$(_context) {
      while (1) {
        switch (_context.prev = _context.next) {
          case 0:
            return _context.abrupt("return", {});

          case 1:
          case "end":
            return _context.stop();
        }
      }
    }, _callee, this);
  }));

  return function fetchPageData(_x) {
    return _ref.apply(this, arguments);
  };
}();

Note that while regeneratorRuntime.mark is marked as #PURE, the call to _asyncToGenerator and the IIFE aren't.

Possible Solution

Generated code should start with:
var fetchPageData = /#__PURE/ function () {

Context

I have a react app which runs on both client and server. However, I have various async functions that only run on the server. I want to have them removed during the minification process.

[babel-bot]

Hey @winstonewert! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
community that typically always has someone willing to help. You can sign-up here
for an invite.

[hakz-00]

Hello, It seems no one has offered to provide a solution. Would you mind if I had a go at it? It is my first time contributing to open software, but I think I can manage something. :)

[nicolo-ribaudo]

@Eric-Zayas Go for it!

[kzc]

@Eric-Zayas Perhaps you could try something like this?

--- a/packages/babel-helper-remap-async-to-generator/src/index.js
+++ b/packages/babel-helper-remap-async-to-generator/src/index.js
@@ -4,6 +4,7 @@ import type { NodePath } from "@babel/traverse";
 import wrapFunction from "@babel/helper-wrap-function";
 import * as t from "@babel/types";
 import rewriteForAwait from "./for-await";
+import annotateAsPure from "@babel/helper-annotate-as-pure";
 
 const awaitVisitor = {
   Function(path) {
@@ -71,4 +72,5 @@ export default function(path: NodePath, file: Object, helpers: Object) {
   path.node.generator = true;
 
   wrapFunction(path, helpers.wrapAsync);
+  annotateAsPure(path.isDeclaration() ? path.get("declarations.0.init") : path);
 }

I didn't run the test suite due to unrelated errors in my environment.

[Andarist]

@Eric-Zayas Have you found some time to work on the issue? Should we keep it as claimed for you? No rush, just let us know :)

[satya164]

I'd like to work on this if you don't have time @Eric-Zayas

[xtuc]

@Eric-Zayas FYI a PR has been merged for this. Sorry if you was working on it but you didn't answer.