We rewrote our Rust WASM Parser in TypeScript – and it got 3x Faster

We rewrote our Rust WASM Parser in TypeScript - and it got 3x Faster

Thesys Engineering Team · Fri Mar 13 2026

We built the openui-lang parser in Rust and compiled it to WASM. The logic was sound: Rust is fast, WASM gives you near-native speed in the browser, and our parser is a reasonably complex multi-stage pipeline. Why wouldn't you want that in Rust?

Turns out we were optimising the wrong thing.

The openui-lang parser converts a custom DSL emitted by an LLM into a React component tree. It runs on every streaming chunk — so latency matters a lot. The pipeline has six stages:

autocloser → lexer → splitter → parser → resolver → mapper → ParseResult

Autocloser : makes partial (mid-stream) text syntactically valid by appending minimal closing brackets/quotes

: makes partial (mid-stream) text syntactically valid by appending minimal closing brackets/quotes Lexer : single-pass character scanner, emits typed tokens

: single-pass character scanner, emits typed tokens Splitter : cuts the token stream into id = expression statements

: cuts the token stream into statements Parser : recursive-descent expression parser, builds an AST

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