Provide type coverage metrics

Search Terms

• type coverage
• flow coverage

Suggestion

I'd love it if TypeScript had a tool to capture type coverage metrics, similar to Flow's Type Coverage feature. That coverage data could be used by IDEs (again, currently supported by Flow) to highlight gaps in type coverage. Basically, it should tell us which parts of our code are being evaluated as any.
Assuming it would work like Flow's feature, this could be a separate tsc option or separate cli command. Theirs works for single files and also offers a batch-coverage command for checking directories / whole projects, etc.

Use Cases

• The Type Coverage metric could be used much like test coverage info, helping us to identify gaps in our typing (particularly unexpected gaps caused by insufficient type definitions from library code).

Examples

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing the Flow Type Coverage and batch-coverage documentation linked in the issue, then examine how TypeScript's tsc options or CLI commands could expose equivalent metrics. Done means reporting which parts of the code are evaluated as any for individual files and directory or project-wide coverage, with IDE-consumable data.