Assigns global properties when defining a function that has a function type argument with default value and rest parameters

[ryokkkke]

Bug Report

• I would like to work on a fix!

Current Behavior
This bug occurrs when transpile TypeScript codes.

1. define a function that has function type argument with default value and rest parameters.
2. define a variable that has same name as the argument function's argument name in the function body.
3. then compile, and 'var' expression in a for sentence is disappearing from output JS codes.
4. JS runtime will throw an error like ReferenceError: _len is not defined because the assignment of a global variable is not allowed with "use strict";.
5. change the BUGGY_ARGUMENT to DIFFERENT_ARGUMENT
6. works fine

Input Code

const func = (
  someFunc: (BUGGY_ARGUMENT: number[]) => boolean = () => false,
  ...restParameters: number[]
) => {
  const BUGGY_ARGUMENT = someFunc(restParameters);
  console.log(BUGGY_ARGUMENT);
};

// output js codes
"use strict";

var func = function func() {
  var someFunc = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : function () {
    return false;
  };
  return function () {
    // THIS '_len' WILL THROW AN ERROR:
    // `ReferenceError: _len is not defined`
    for (_len = arguments.length, restParameters = new Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
      restParameters[_key - 1] = arguments[_key];
    }

    var BUGGY_ARGUMENT = someFunc(restParameters);
    console.log(BUGGY_ARGUMENT);
  }.apply(null, arguments);
};

Expected behavior/code

const func = (
  someFunc: (DIFFERENT_ARGUMENT: number[]) => boolean = () => false,
  ...restParameters: number[]
) => {
  const BUGGY_ARGUMENT = someFunc(restParameters);
  console.log(BUGGY_ARGUMENT);
};

"use strict";

var func = function func() {
  var someFunc = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : function () {
    return false;
  };
  // THIS _len WORKS FINE!!
  for (var _len = arguments.length, restParameters = new Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
    restParameters[_key - 1] = arguments[_key];
  }

  var BUGGY_ARGUMENT = someFunc(restParameters);
  console.log(BUGGY_ARGUMENT);
};

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

• Filename: .babelrc

{
  "presets": [
    ["@babel/env", {
      "targets": {
        "browsers": ["last 2 versions"]
      }
    }],
    "@babel/typescript"
  ]
}

Environment

• Babel version(s): 7.7.7
• Node/npm version: Node 10
• OS: macOS 10.14.6
• Monorepo: no
• How you are using Babel: cli

Possible Solution

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

This also occurs on online playground like the following

https://babeljs.io/repl#?browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=MYewdgzgLgBAZgVzMGBeGAKAUDGEQC2ApgGJLABcmAQgKoDi9AmgPoCCASvbQLICiAOQAqVMAgIAjIgCcA2gF0AlGgB8MCSBAAbIgEMwaTMtRq4urRCIAaHDAB0D6UWgAFXdN3EoMiKPFS5eSxjNQBvW1BIWDpGVk5ufmFDfGIyZAwnV3dPIm9pCEUAbgjwfB07LRAAcwwY5nYuXkEhIqwAX2KsRHSMEJgoaQQiVqA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2%2Ctypescript&prettier=false&targets=&version=7.7.7&externalPlugins=

[babel-bot]

Hey @ryokkkke! 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]

Closing this as duplicate of #10798.

@ryokkkke Thank you! It is a nice report.

[ryokkkke]

@JLHwung
Thank you for checking!
I'm going to watch conversations on that issue from now on:)