Suggestion: Allow Types in JS Files

I have a large Angular2 project written in JavaScript, in which we use transform-flow-strip-types in order to allow Angular2 dependency injection to work from JavaScript.

This is currently an error in a JavaScript file compiled with tsc:

class MyClass {
  prop:string;                   //err: 'types' can only be used in a .ts file
  constructor(x: string) {       //err: 'types' can only be used in a .ts file
  }

  someMethod():string {        //err: 'types' can only be used in a .ts file
  }
}

I would like an allowTypesInJsFiles compiler option which would allow us to compile JavaScript files which contain types. All of the above errors would be valid if the flag is set. This would allow us to use Angular2 DI from JavaScript files.

I have a branch with a simple implementation which I have verified to work.

Is this a change which could be accepted? If so I will add tests and sign the CLA.

I think it would be very useful for people migrating large JavaScript projects to TypeScript.

[cfaester]

I hate to bump an old issue, however, as issues like #30115 spring up, and this one is still open, I want to ask what’s the status on this?

I believe adding what @sberan suggests would greatly support the adoption of TypeScript when you are only using tsc as a type-checker, and running the code through Babel. It would also be a big help when migrating large code bases.

[liamness]

This seems like an old issue, and it may be that there is a resolution that I am missing, but if not I would like to request this feature addition. It’s a little confusing that allowJs and checkJs exist as options, but that you nonetheless must use a .ts extension to suppress the errors that result. Makes those options seem a little redundant. I am using many non-standard JS syntaxes and transforms in my projects, through Babel, but Typescript is the only one which has this requirement.