The scope of the arguments default value is different from what was expected in ES2015 (参数默认值的作用域与 ES2015 的预期结果不同)

[Yangfan2016]

Bug Report

Current Behavior
The scope of the arguments default value is different from what was expected in ES2015

Input Code

online view

This is source code of ES2015:

let foo = 'outer';

function bar(func = () => foo) {
  let foo = 'inner';
  console.log(func());
}

bar(); // outer

The result of run it:

outer

This is transform code by babel:

"use strict";

var foo = 'outer';

function bar() {
  var func = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : function () {
    return foo;
  };
  var foo = 'inner';
  console.log(func());
}

bar(); // outer

The result of run it:

inner

Expected behavior/code

Expect the results of the Babel conversion to be consistent with ES2015 performance

Babel Configuration (.babelrc, package.json, cli command)

Environment

• Babel version(s): [v7.4.4]
• Node/npm version: [Node 8.9/npm 5]
• OS: [Windows 10]
• Monorepo: [no]
• How you are using Babel: [babeljs.io]

Possible Solution

"use strict";

var foo = 'outer';

function bar() {
    var func = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : function() {
        return foo;
    }
    ;
    (function() {

        var foo = 'inner';
        console.log(func());

    }())
}

bar();

Additional context/Screenshots
Add any other context about the problem here. If applicable, add screenshots to help explain.

[babel-bot]

Hey @Yangfan2016! 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]

Hi 杨帆,

Thanks for the reproducing examples. It helps a lot narrow down the problem.

The possible solution you mentioned above is promising. In fact there is a iife state in the transform-parameters to determine whether the whole function body should be wrapped into an extra closure.

It is interesting to find that there was a similar bug people had already solved here, so as a workaround, you can replace

function bar(func = () => foo) {
}

to

function bar(func = () => eval("foo")) {
}

And babel will work this time. I admit that it is ugly hack and the bug should be fixed in transform-parameters. I would look into this in a couple of days.