Noticed when tried to use typescript-rails, actual issue is apparently within this gem.

typescript-ruby/typescript-rails#42

Based into documentation (https://www.typescriptlang.org/docs/handbook/compiler-options.html), "--out" options is deprecated and should use "--outFile". Did quick hack to test that, and didn't notice any change in behavior (output_js still became empty, and "/tmp" appears to contain generated temp files instead).