Hmm I just found another project that tries to accomplish this - I haven't tried it though.
https://www.npmjs.com/package/es-feature-detection

How did you write the script you tested with @dotnetCarpenter?
What version of ES Check were you using?

^ Questions asked to ensure it's not a potential feature that needs to be added.

ES-check is built on acorn.js. I trust it. There are new features that are popping up though. So perhaps feature detection will become a thing that es-check supports.

Es-feature-detection provides some interesting things that could potentially be provided by ES-Check. Thanks for sharing.

The ones for globals like Map and Set will require some keeping track of the variables. For a single file, this can probably be done by walking the syntax tree with acorn-walker or something similar and looking for uses while making sure that they are not shadowed by something else in the scope. Stuff in ESTree like Function, BlockStatement, WithStatement, VariableDeclaration, AssignmentExpression, Expression will need to be checked.

The story about new methods like Array.prototype.flatmap is a bit more frightening, since they are going to require type deduction and checking. An example of doing so is in the no-for-in-array rule of typescript-eslint. (Some more complex ones like await-thenable requries the tsutils package.) Acorn will not do: this linter uses a ESTree parser generated by consuming typescript AST to achieve that. On the other hand typescript compiler may be good for keeping track of global variables for you too, but it sure is a lot slower.