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The Type Linter for JS.


TypL provides optional type annotations for JS programs, so you can verify that you haven't mixed incompatible value-types in any operations (which can cause bugs!). However, TypL takes a different approach from the more well-known TypeScript and Flow tools.

As a quick glance at the differences:

  • not a typed-language variant of JS, but rather a (type) linter in the truest sense: TypL checks code against a set of opinions (that you control!) about how types should be treated in your program; with a heavy emphasis on type inferencing, you can run type linting on existing JS programs without any code changes

  • for type annotations, uses only standard valid JS syntax (ES6 template tags), so type-annotated code can be executed without any compilation step if desired (as long as the runtime library is present)

  • shifts focus from "typing your variables" to "typing your values and expressions"; variables optionally get assigned "implied types" from the annotated value-types

  • provides compile-time static type checks as well as runtime dynamic type checks (assertions), both of which are optional

  • completely configurable (like ESLint), so you're always in control of what is reported as a type error or not -- for example, you decide if some specific type conversion/coercion is allowed, etc

TypL is still in early development. For more information, please see:


bin/typl --file=./some-code.js


node ./lib/cli.js --file=./some-code.js


npm test

Project Champions

I would like to thank the following people for their generous sponsorship as a project champion. You are awesome!


All code and documentation are (c) 2019 Kyle Simpson and released under the MIT License. A copy of the MIT License is also included.