TypeScript

The TypeScript engine accepts .ts, .mts and .tsx files. Types are stripped first (with sucrase), then the JavaScript is analysed and rewritten, so the output is JavaScript. With several files and -o dir/, the outputs are named .js, .mjs and .jsx.

The Rust engine does not take TypeScript: it refuses these files with a message. Use the TypeScript CLI for them, or strip the types first.

Viscosity doesn’t rely on type annotations for safety: x: number doesn’t stop a caller passing a string at run time. It proves numeric-ness from the program itself.

A related measured finding: TypeScript class field declarations (x: number;) become real class fields that start as undefined, which made a hot update loop 3–7× slower in V8 than object literals. The book’s chapter 22 explains why.