Transpiling

Transpiling, or source-to-source compiling, is the process of transforming source code written in one programming language into equivalent source code in another language at a similar level of abstraction. The goal is to preserve behavior while adapting syntax, language features, or platform constraints.

It is used to enable newer language features in environments that do not support them, to translate code between ecosystems, or to optimize tooling, readability, or performance characteristics. A transpiler typically focuses on transforming code rather than producing optimized machine code.

How it works: a transpiler parses the input, builds an intermediate representation such as an abstract syntax tree, applies transformations, and emits code in the target language. The mapping is often complex and may require semantic preservation and careful handling of language features to maintain equivalence.

Common use cases include TypeScript to JavaScript, CoffeeScript to JavaScript, JSX/TSX to plain JavaScript, ESNext to older JavaScript via Babel, or Dart to JavaScript. CSS preprocessors like Sass or Less are sometimes described as transpilers because they produce standard CSS from higher-level syntax. Some languages also compile to another high-level language, such as Kotlin/JS or Scala.js.

Relation to compiling: unlike compilers that emit machine code or bytecode, transpilers produce source code in a different language. The term transcompilation is often used as a synonym.

Limitations can include semantic gaps between languages, debugging challenges, and potential inefficiencies in the generated code.