Invalid output resulting in ReferenceErrors with plugin-proposal-optional-chaining

[felixfbecker]

Bug Report

• I would like to work on a fix!

Current behavior

When a nested optional chaining expression is found inside a parameter position (a parameter default value, or default value inside an object or array destructuring), a var declaration is added to the function body. That variable declaration is not in effect when evaluating parameter default values and causes a ReferenceError in strict mode when the function is called.

• REPL

To repro, you can use the Babel CLI to compile the code below and run it with Node: babel test.js | node. Error output:

const foo = (param = (_obj$prop = obj.prop1) === null || _obj$prop === void 0 ? void 0 : _obj$prop.prop2) => {
                                ^

ReferenceError: _obj$prop is not defined
    at foo ([stdin]:5:33)
    at [stdin]:11:1
    at Script.runInThisContext (vm.js:132:18)
    at Object.runInThisContext (vm.js:315:38)
    at Object.<anonymous> ([stdin]-wrapper:10:26)
    at Module._compile (internal/modules/cjs/loader.js:1256:30)
    at evalScript (internal/process/execution.js:98:25)
    at internal/main/eval_stdin.js:29:5
    at Socket.<anonymous> (internal/process/execution.js:211:5)
    at Socket.emit (events.js:326:22)

Input Code

'use strict'
const obj = {}
const foo = (param = obj.prop1?.prop2) => {
  console.log(param)
}
foo()

Output Code

'use strict';

const obj = {};

const foo = (param = (_obj$prop = obj.prop1) === null || _obj$prop === void 0 ? void 0 : _obj$prop.prop2) => {
  var _obj$prop;

  console.log(param);
};

foo();

Expected behavior
Should not throw a ReferenceError, should log undefined to the console.

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Filename: babel.config.js

See REPL link above, or just add the plugin-proposal-optional-chaining (doesn't matter whether loose or not, happens in both modes)

{
  "plugings": [
    "@babel/plugin-proposal-optional-chaining"
  ]
}

Environment

• Babel version(s): 7.11.5
• Node/npm version: v14.7.0
• OS: macOS
• Monorepo: [e.g. yes/no/Lerna]
• How you are using Babel: CLI for repro

[babel-bot]

Hey @felixfbecker! 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."

[JLHwung]

Duplicate of #11166