Static typing in JavaScript

What is static typing?

JavaScript has types but they are only known at runtime
Static typing adds support for type checking to a language at compile time
Can you look at a random piece of code and tell what might be the type of a variable before it runs?
- You can sometimes - it's called type inference
- But cannot in many cases - where one might need explicit type definitions
Enforces a contract on the type of shape a value can take beforehand

Thinking in types first
- Domain driven development with types
Early bug detection
Some unit tests are no longer needed
Documentation which can be trusted at all times
Refactor with confidence
- Both small and full architecture refactors
Editor/IDE support
- More information about a variable
- Better and more intelligent autocomplete
- Automatic refactorings
In a big team, defines interfaces which everyone can understand
Adds a layer of safety and correction

Higher upfront cost
- Have to think about types before you see anything on the screen
Learning curve
- Thinking in types is a skill which needs time to master
Third party libraries might not be typed
- Even if they are typed, the types might be very shallow (e.g. "any" type for everything)
Adds to development time
For team new to react, better investments might be in
- react patterns
- redux patterns
- testing strategies

Typescript
- More mature system
- Has better support for third party typed libraries
- Has better editor support system
  - Specially with vscode
- Needs bigger commitment - all or none
Flow
- Is new but allows gradual introduction to current system (not a problem we have right now)
- Has problem with a lot of breaking changes with every new release
Statically typed languages which compile down to javascript - Elm, Purescript, Reasonml