Type-checking

with examples in TypeScript

Dapeng Li

Agenda

  • Type-checking

  • Custom types with TS

  • Some of TS' caveats

  • Summary

TypeScript

Reports type errors

JavaScript

Doesn't JavaScript have types?

And report type errors?

TS compilation

Type system *

  • Dynamic

    • type-checking at runtime

  • Static

    • type-checking at compile time

Source-to-source compile = transcompile = transpile **

** https://en.wikipedia.org/wiki/Source-to-source_compiler

* https://en.wikipedia.org/wiki/Type_system

Samurai

Ninja

* https://ninjasvssamurai.weebly.com/comparing.html

OO Constructs

Class = state + behaviour

Inheritance

Inheritance

Inheritance

  • models is-a relationship

  • a strong form of coupling

  • single inheritance

Interface: contract

Interface

  • Models can-do relationships

  • contract w/o impl.

  • weaker coupling than class

  • class can impl. 1+ interfaces

Mixin: contract + impl.

Mixin in TypeScript: step 1/2

Mixin (cont.)

Mixin in TypeScript: step 2/2

Mixin

  • interface w/ impl.

  • best of both worlds?

  • be ware of the coupling

Generics

operation doesn't care about type

Generics

preserving type

Generic constraints

Caveats & "False Friends"

  • Type-checking rules

  • Variance

Compatible types?

// This is TypeScript
class Cat { name: string; run() {console.log("cat runs");} }
class Dog { name: string; run() {console.log("dog runs");} }

let cat: Cat = new Cat();  // fine
let dog: Dog = new Dog();  // fine

dog = cat;  // what about this?

Type compatibility *

  • Structural

    • check structures

  • Nominal

    • check names/declarations

* https://en.wikipedia.org/wiki/Type_system

Structural TS

Structural TS

  • Structural on public members

  • Nominal on private/protected members *

* https://github.com/Microsoft/TypeScript/wiki/FAQ#when-and-why-are-classes-nominal

Java: too strict for Catdog

Variance *

  • Dog[] and Animal[]

  • (Dog -> void) and (Animal -> void)

* https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

Dog is compatible with Animal

Variance *

  • Covariant

  • Contravariant

  • Invariant

  • Bivariant **

* https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

let animal: Animal = new Dog();
let animals: Animal[] = [new Dog()];
let dogs: dog[] = [new Animal()];
let animals: Animal[] = [new Dog()];
let dogs: dog[] = [new Animal()];
// no code here...

** https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

Variance in TS

* https://www.typescriptlang.org/docs/handbook/type-compatibility.html

TypeScript is helpful

Tooling support

TypeScript is helpful

Think with constraints

  • Be more precise

  • Keep things separated

TypeScript is helpful

Stepping stone to OO work

Community is growing

* http://www.benfrederickson.com/ranking-programming-languages-by-github-users/

TypeScript is young

  • Lacks guidance

  • Can be confusing

Personal suggestions

  • Learn and use it

  • Be selective

  • Take it slowly

Takeaway

  • Dynamic vs. Static type-checking

  • Structural vs. Nominal typing

  • Class, Interface, Mixin and Generics

Questions?

class-based languages

*

* Classes, Interfaces, Mixins & Generics

Type-checking with examples in TypeScript

By Dapeng Li

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Type-checking with examples in TypeScript

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