Arguments undefined when spreading args in async arrow function (T7329)

[babel-bot]

Issue originally made by @Lapixx

Bug information

• Babel version: 6.7.7
• Node version: 5.3.0
• npm version: 3.3.12

Options

ES2015 and Stage 0 preset.

Input code

// modified from kadirahq/graphql-errors

const modifyResolver = (field, handler) => {

  const originalResolve = field.resolve;

  field.resolve = async (...args) => { // Workaround 1: use `async function(...args) {`

    try {
      // const b = args; // Workaround 2: Access args -- even when unused.
      const res = originalResolve(...args); // Problem: arguments is undefined here
      return await Promise.resolve(res);
    }
    catch (err) {
      throw handler(err);
    }
  };
};

Description

Basically, when I try to define a function like so:

const myFunc = async (...args) => {
    otherFunc(...args);
};

The arguments are undefined. When you look at the output you can clearly see that arguments gets assigned outside of the function scope. I'm not sure why that happens. Removing async fixes the problem, using the function keyword fixes the problem, and (my favourite), assigning args to a value (or otherwise accessing it) also fixes the problem.

[[ https://babeljs.io/repl/#?evaluate=true&lineWrap=false&presets=es2015%2Creact%2Cstage-0&experimental=true&loose=false&spec=false&code=%2F%2F%20modified%20from%20kadirahq%2Fgraphql-errors%0A%0Aconst%20modifyResolver%20%3D%20(field%2C%20handler)%20%3D%3E%20%7B%0A%20%20%0A%20%20const%20originalResolve%20%3D%20field.resolve%3B%0A%20%20%0A%20%20field.resolve%20%3D%20async%20(...args)%20%3D%3E%20%7B%20%2F%2F%20Workaround%201%3A%20use%20%60async%20function(...args)%20%7B%60%0A%0A%20%20%20%20try%20%7B%0A%20%20%20%20%20%20%2F%2F%20const%20b%20%3D%20args%3B%20%2F%2F%20Workaround%202%3A%20Access%20args%20--%20even%20when%20unused.%0A%20%20%20%20%20%20const%20res%20%3D%20originalResolve(...args)%3B%20%2F%2F%20Problem%3A%20arguments%20is%20undefined%20here%0A%20%20%20%20%20%20return%20await%20Promise.resolve(res)%3B%0A%20%20%20%20%7D%0A%20%20%20%20catch%20(err)%20%7B%0A%20%20%20%20%20%20throw%20handler(err)%3B%0A%20%20%20%20%7D%0A%20%20%7D%3B%0A%7D%3B | REPL with sample code ]]

[danharper]

Still an issue. Minimal input:

const modifyResolver = () => {
  return async (...args) => {
    //args;
    y(...args);
  };
};

Output:

"use strict";

var _arguments = arguments;

function _asyncToGenerator(fn) { return function () { var gen = fn.apply(this, arguments); return new Promise(function (resolve, reject) { function step(key, arg) { try { var info = gen[key](arg); var value = info.value; } catch (error) { reject(error); return; } if (info.done) { resolve(value); } else { return Promise.resolve(value).then(function (value) { step("next", value); }, function (err) { step("throw", err); }); } } return step("next"); }); }; }

var modifyResolver = function modifyResolver() {
  return function () {
    var _ref = _asyncToGenerator(regeneratorRuntime.mark(function _callee() {
      var _args = _arguments;
      return regeneratorRuntime.wrap(function _callee$(_context) {
        while (1) {
          switch (_context.prev = _context.next) {
            case 0:
              //args;
              y.apply(undefined, _args);

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

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

(args -> _args references arguments from the outer scope).

---

Seems related to #4219?

[resistdesign]

This is still a big problem, see this REPL example: https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=latest%2Creact%2Cstage-0&code=const%20x%20%3D%20async(...arg)%20%3D%3E%20%7B%0A%20%20console.log(...arg)%3B%0A%7D%3B%0A%0Ax(44)%3B%0A

[gajus]

@resistdesign

This is still a big problem, see this REPL example: https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=latest%2Creact%2Cstage-0&code=const%20x%20%3D%20async(...arg)%20%3D%3E%20%7B%0A%20%20console.log(...arg)%3B%0A%7D%3B%0A%0Ax(44)%3B%0A

It looks like the issue is only present once applying the "latest" preset. It works as expected with just the async to generator transpiler. See

https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=stage-0&code=const%20x%20%3D%20async%20(...args)%20%3D%3E%20%7B%0A%20%20console.log(...args)%3B%0A%7D%3B%0A%0Ax(44)%3B%0A

[resistdesign]

@gajus Ah, you're right! Good call!

[aaronang]

Closing this issue because it is a duplicate of #4219. Thank you for taking the time to report though 😬