Making Typescript Stick

Make it stick book --> using to write onboarding

https://www.typescript-training.com/course/making-typescript-stick

// @errors: 2322 2320
let first: string & number;
let second: String & Number;

first = "abc";
second = "abc";
second = new String("abc");

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• When using the primitive types string and number we can see that the union of these two types results in a never. In other words, there is no string that can be also regarded as a number, and no number that can also be regarded as a `string

• When using the interface types String and Number, we can see that the union does not result in a never

Use the lowercase types as much as possible. They are primitives.

What order do logs print?

function getData() {
  console.log("elephant")
  const p = new Promise((resolve) => {
    console.log("giraffe")
    resolve("lion")
    console.log("zebra")
  })
  console.log("koala")
  return p
}
a...

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dog, cat, elephant, giraffe, zebra, koala, lion, moose

Variadic Tuple Types

enum Sandwich {
  Hamburger,
  VeggieBurger,
  GrilledCheese,
  BLT
}
type SandwichOrder = [
  number, // order total
  Sandwich, // sandwich
  ...string[] // toppings
]


// ---cut---
/**
 * return a...

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Example of cleaning types: https://github.com/ReactiveX/rxjs/pull/5859/files

Error type unknown

function somethingRisky() {}
// ---cut---
try {
  somethingRisky();
} catch (err: unknown) {
  if (err instanceof Error) throw err;
  else throw new Error(`${err}`);
}

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interface vs. type

// @errors: 2300
type Color = {
  red: number;
  green: number;
  blue: number;
};

interface Color {
  alpha: number;
}

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Interfaces are open, types are not. Two interfaces would compile.

Learning

https://github.com/type-challenges/type-challenges

// @errors: 2344
type Expect<T extends true> = T
type Equal<X, Y> =
(<T>() => T extends X ? 1 : 2) extends
(<T>() => T extends Y ? 1 : 2) ? true : false

type NotEqual<X, Y> = true extends Equal<X, Y>...

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