Static Javascript typing with Flow
Flow is a static type checker for JavaScript. It can be used to catch common bugs in JavaScript programs - such as silent type conversions, null dereferences and so on - often without requiring any changes to your code. It also adds type syntax to JavaScript, so that developers can express invariants about their code and have them maintained automatically.
This is a contrived toy app which pretends to load an "order" from on online store in parallel and then combines the parts and prints out an order breakdown.
Flow types are removed when compiling, they are only used for development.
Babel can be used to remove flow types but nothing else:
babel -l flow demo.js > demo-plain.js
Running the demo
Install flow. (brew install flow)
Run flow. It should report "No Errors"
Edit the code and introduce a typo like order.pricetotal and rerun flow.
Editor support & more
Editor support is limited but Facebook has a plugin for Vim.
Babel has native support for flow.