Demo of a typed JavaScript project using Flow.
While JavaScript has a counterintuitive typesystem (obligatory meme), it does not mean it is impossible to typecheck programs written in it. It is true that humans find type coercion and comparison rules difficult and irrational, that is because they tend to reason in terms of arithmetic systems they are familiar with, e.g. integers or real numbers.
Extending the JavaScript typesystem with rules that make it not just easier to work with but also sound is the goal of projects like Flow. The Flow typesystem is described on ArXiv. TypeScript is another example, albeit with a formally weaker typesystem, but equally great for development.
One focus of this repository is on typing data structures and algorithms implemented using plain JavaScript techniques, with Flow annotations.
It also focuses on testing. Typecheckers can decide if the implementation is safe—as in, every evaluation step is operating only on values of the expected type (type soundness)—but not whether the implementation is actually conforming to the algorithm specification (unless the typechecker also happens to be a theorem prover, so that type definitions and algorithm specification are the same). Testing is necessary to have a certain degree of confidence (but no guarantee) in the soundness of the implementation.
Every module has three parts:
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JavaScript implementation (*.js): Direct implementations of some generic data structures in plain JavaScript (ES8) with Flow annotations.
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Runtime tests (*.spec.js): Unit tests. These are typechecked first, then types are stripped using the flow-strip-types Babel transform before running Jest.
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Compile-time tests (*.flowcheck.js): Compile-time checks for Flow types. Never executed in runtime, only automatically checked by Flow. It contains error expectations that raise warnings if not met, as well as raising regular type errors.
Not meant for actual usage (production or not). The data structures are by default only used in tests, but it is possible to just import them in any JavaScript project after stripping all type annotations with Babel.
First, install the dependencies: npm i && flow-typed install jest.
- Typecheck:
npm run flow - Test:
npm run test
This software is released under the MIT license. See LICENSE for more informations.