Static vs Dynamic
Typing

Dynamic vs Static Typed

It's more of a philosophical question
The issue of types predates computers
The mathematical problem of ambiguous statements
A solution was the notion of types

What we have learned over the last half-century is what not to do.

Clean Architecture by Uncle Bob

Studies

Statically typed languages in general are less defect prone than the dynamically typed ones.

Studies:

A Large Scale Study of Programming Languages and Code Quality in Github
Software Defect Origin and Removal Methods, by Capers Jones, 2012

Static types addons

Dynamic languages with static types addons:

Python - Pyre (Facebook, Instagram)
PHP 5 - Hack (for HHVM, Facebook)
PHP 7 - Scalar Type Declarations (official)

Advantages of Static Types

Enables better tool support
- safe refactoring in a big codebase
- better autocompletion
- code exploration
The best form of documentation
- less verbose than using comments
Types make abstractions explicit
Provide well defined boundaries between sub-systems

Why TypeScript

Popular with developers
Developed by Anders Hejlsberg
Great tooling
Great 3rd party support
- code editors, compilers, linters, and so on
Very good support for React and JSX
Used by big projects
- Angular 2, Dojo

Types in TypeScript

TypeScript is intentionally and strictly a superset of JavaScript
TypeScript is optionally typed by default

Types:

can be implicit -> inference
can be explicit -> by adding type annotations
are structural -> checks the structure of a construct
can be ambiental
- means declared in a different place than the code
- can be added to existing code (like libraries)

Other TypeScript Features

JavaScript code is valid TypeScript code

Features:

interfaces
enums
hybrid types
generics
union and intersection types
access modifiers

Costs

there is a learning curve
takes time to convert an existing code base
have to rely on 3rd party packages for types
types can be a bit verbose
types can make the code more rigid

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