JavaScript

TypeScript

TypeScript is JavaScript with Type checking. A superset of JS. First learned in CS349.

From CS349

• Main feature added: static types
• typescript is statically typed
  • performs type checking at compile time
  • once we assign a variable a type, we can’t change it

”JavaScript with Type Checking”

• TypeScript is a superset of JavaScript
• Developed by Microsoft starting in 2012
  • Led by Anders Hejlsberg (creator of C# and Turbo Pascal)
  • Took off in 2014 (when Angular 2 chose TypeScript)
• Main feature is adding static types
  • enables type checking
  • enables code completion

TypeScript is transpile to JavaScript in order to run in the browser. → remember the browser only executes JavaScript

• Our Vite setup lets you debug TypeScript source
  • but you’re debugging JavaScript from a TypeScript “source map”
• Most TypeScript tutorials show how to install the tsc compiler
  • Our Vite dev environment already transpiles to TypeScript, there is no need to install/call tsc
• ES Module

Types

• TypeScript types are annotations
  • checked at “compile” time, not run time

Avoid the any type

Caution: implicit type inference can result in the any type!

Use CamelCase for type aliases (I always forget…)

Variable Typing and the Any Type

• When you don’t give the variable an explicit type annotation, or an initial value so it can do implicit type inference, it’s the any type
• The any type turns off typescript’s type checking

Array, Object, Function, Interface Typing

Structural Type Checking

• Only the structure of a type matters
• If an object has the parameter we need, in the order we need it, nothing else is relevant

Optional Parameters

• Denoted by ?
• Type narrowing is usually required

Enums vs. Literal Types