Issue with processing typescript files with certain generic expressions.

[mmorton]

When calling transpileModule on a plain .ts file, with certain generic expressions and when jsx is set to ts.JsxEmit.Preserve, incorrect JS will be emitted. For example:

const ts = require('typescript');

const contents = `
const something = <T>(a: T) => {
}
`;

const processed = ts.transpileModule(contents, {
    compilerOptions: {
        target: ts.ScriptTarget.Latest,
        jsx: ts.JsxEmit.Preserve
    }
});

console.log(processed.outputText);

will emit:

const something = <T>(a: T) =>
</>;

Which will cause errors when Babylon tries to parse it.

[skolmer]

thanks for the pull request! to understand this right, is the preserve flag treating simple arrow functions that do not return jsx as react stateless components?

[mmorton]

It's treating the generic type expression <T> as the start of a JSX element.

[mmorton]

To use those in a .tsx file, you have to either have multiple generic parameters, or use qualifiers. But since this is in a normal .ts file, it should not matter (and does not matter when using tsc to build everything). The only issue is here, that transpileModule doesn't have that same information about file extensions that is there when building normally.

[skolmer]

ok, I think this fix will work fine for 99% of typescript user. If for some reason someone is writing jsx in .ts files this will be an issue but there is still the possibility to write your own preprocessor if you want a custom typescript processing behavior. so I merged this an it is now online in 2.4.3, thanks!