Invalid warning for not declaring 'corejs' option when using 'useBuiltIns': 'false'

[JMarkoski]

Bug Report

• I would like to work on a fix!

Current Behavior
When using @babel/preset-env and setting useBuiltIns: 'false' and not declaring a corejs option, the following warning occurs:

WARNING: We noticed you're using the useBuiltIns option without declaring a
core-js version. Currently, we assume version 2.x when no version
is passed. Since this default version will likely change in future
versions of Babel, we recommend explicitly setting the core-js version
you are using via the corejs option.\n" +
You should also be sure that the version you pass to the corejs
option matches the version specified in your package.json's
dependencies section. If it doesn't, you need to run one of the
following commands:
npm install --save core-js@2 npm install --save core-js@3
yarn add core-js@2 yarn add core-js@3

First, I don't know if this should be considered a bug, because the problem occurs only when setting the useBuiltIns option to the string 'false' and not false. But if you set it accidentaly to a string value I think It should behave the same as when setting it to a boolean one.

Expected behavior/code
The warning should not occur.

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

• Filename: babel.config.js

{
  presets: [
    "@babel/env",
    {
      useBuiltIns: "false"
    }
  ]
}

Environment

• Babel version: 7.9.0
• Babel preset-env version: 7.9.0
• Node/npm version: 10.17.0
• OS: Ubuntu 18.04.3 LTS
• Monorepo: [e.g. yes/no/Lerna]
• How you are using Babel: [loader]

Possible Solution
Upon inspecting the @babel/preset-env source code, in the file normalize-options.js in the function normalizeCoreJSOption the following check is used

if (useBuiltIns && corejs === undefined) {

Using a more thorough check for useBuiltIns will fix the warning.

[babel-bot]

Hey @JMarkoski! 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."

[nicolo-ribaudo]

We should throw an error about "false" not being a valid value instead.

[JMarkoski]

Yeah, that's definitely way more reasonable to do. I spent hours digging what the problem could be, and rereading the docs about useBuiltIns, thinking I must have misunderstood something in how the option works. So I think it's reasonable to add a better error handling there.

[JMarkoski]

Also, there are many places where the same situation can happen, so I think it should be either done everywhere or nowhere at all.
I don't know how it's handled in other places if you provide invalid configs, but I suppose a config should be checked against an api schema to avoid invalid configs and provide better error messages.

[JLHwung]

@JMarkoski

preset-env uses invariant to validate useBuiltIns

babel/packages/babel-preset-env/src/normalize-options.js

Line 187 in 9444713

UseBuiltInsOption[builtInsOpt.toString()] === UseBuiltInsOption.false,

It seems to me that it suffices to simply this into

builtInsOpt === false

since practically "false" will never be allowed as a different option than false.

Can you submit a PR? Thank you!

[JMarkoski]

@JLHwung Yes, I will submit a PR. Also, the same thing should be done with

babel/packages/babel-preset-env/src/normalize-options.js

Line 171 in 9444713

ModulesOption[modulesOpt.toString()] === ModulesOption.false,

I will add a fix for that in the PR as well.
Thanks.