I have recently released the initial version of a TypeScript port of StringTemplate4. The repository can be found at
https://github.com/mike-lischke/stringtemplate4ts.
Although it primarily focuses on Node.js as a runtime due to its use of file access, it should be possible to run it on web browsers with the usual bundlers and by avoiding imports that require file imports.
In terms of usage, it is very similar to the Java version, so you should feel comfortable with it right away.