Add .ts and .tsx support on ESLint

[brendanoffer]

Hi, our project is using ESLint to lint our .ts and .tsx files. We are doing this because TSLint is being deprecated and moved to @typescript-eslint.

Currently, the hound ESLint parser is configured to parse .js .es6 .jsx .vue files. Would you be open to either including .ts and .tsx files in the FILE_REGEXP or a PR to extend hound's configuration to accept more files extension types?

As an aside, eslint itself only lints js files by default. In order to lint your files an --ext=.ts,.tsx argument is necessary for ESlint to scan typescript files.

[gylaz]

Yes, a PR for this would be welcome.

A few questions that I have regarding support of TS files with Hound and ESLint.

• What happens when we run eslint foo/bar/baz.ts? (this is how Hound actually lints each file)
• How do we know if a user wants their TS files linted with ESlint? Can we lint them anyway if typescript-eslint is not present?

[brendanoffer]

1. If hound passes a .ts file to eslint it will lint the file regardless of extension. The issue is that eslint (or hound) will not select or lint these files by default. For example something like eslint components/ would lint all js and jsx files but not ts. Conversely, if we ran eslint --ext=.ts,.tsx components/ it would run against the ts files.

2. a. I believe you'd have to make the file types (extensions) configurable by the user.
   b. You can lint the ts files without typescript-eslint. The linter would run against the file based on the configured eslintrc file. I don't think you would want to prevent this from happening as its possible some users would want to customize which linters and rulesets they use.

[brendanoffer]

I believe this PR will force Hound to scan the files. If users do not want the files scanned they can use eslint built in functionality with the eslintignore file

#1738

[gylaz]

The related PR was merged.