GtkSourceView has built-in support for TypeScript.

Vala Tester spawns a command line to run Vala as a REPL.

If we have node-typescript as a dependency, we can run the compiler and still use just the output JavaScript file in the WebView. The issue would be compiler errors. The compiler seems to still compile the TypeScript to JavaScript, but still outputs errors.
Errors get output via stdout (1> filename), not stderr; we can redirect that to a file named error.log, test to see if it has any contents and, if it does, show that on the WebView instead of using the JavaScript file.