workshop.ts

let hello = "a string";

hello = 5;

const shouldBeAString: string = true;

function addOne(x: number) {
  return x + 1;
}

const four = addOne(3);


const notANumber = addOne("hello");


function timesThree(x) {
  return x * 3;
}

timesThree("hello");

const timesFour = (x: number) => x * '4';




/*
 * ======================================================
 * TODO: Update Earthlings to allow these values:
 * ======================================================*/
type Earthling = {};
let dog: Earthling = { type: "animal", name: "Fido" };
let cat: Earthling = { type: "animal", name: "Suki" };
let zucchini: Earthling = {
  type: "vegetable",
  name: "Zucchini"
};
let rose: Earthling = { type: "vegetable", name: "Rose" };
let quartz: Earthling = { type: "mineral", name: "Quartz" };
let diamond: Earthling = { type: "mineral", name: "Diamond" };

/* typings:expect-error */
const invalidEarthling: Earthling = { type: "martian", name: "Quux" }
/* typings:expect-error */
const invalidEarthling2: Earthling = { type: "plutonian" }

/*
 * ======================================================
 * TODO: Update Aliens to allow these values:
 * ======================================================*/
type Alien = {};

let alienBlaxnor: Alien = {
  name: "Blaxnor",
  homePlanet: "Jupiter",
  phaser: true
};
let alienXanter: Alien = {
  name: "Xanter",
  homePlanet: "Mars",
  phaser: false
};

/*
 * ======================================================
 * TODO: Confirm that EarthlingOrAlien allows and disallows 
 * the following values
 * ======================================================*/
type EarthlingOrAlien = Earthling | Alien;

let person: EarthlingOrAlien = { type: "animal", name: "Fernando" }
let alien: EarthlingOrAlien = { name: "Mac", homePlanet: "Pluto", phaser: true }

// typings:expect-error
let star: EarthlingOrAlien = "Sirius";

// typings:expect-error
let galaxy: EarthlingOrAlien = {
  type: "LocalGalaxy",
  name: "Milky Way"
};

// typings:expect-error
let asteroid: EarthlingOrAlien = {
  homePlanet: false,
  name: "Asteroid"
};


/**
 * If we have a value of a union type, we can access common properties
 */
function printName(creature: EarthlingOrAlien){
  console.log(creature.name)
}

 /**
  * But we cannot access properties that aren't common to all constituents
  */
function printType(creature: EarthlingOrAlien) {
  // typings:expect-error
  console.log(creature.type);
}




/*
 * Intersections allow us to combine type definitions to
 * create a single type with all the attributes of both types.
 * This is useful when we have some set of properties that are
 * shared among many different types. */
type Cat = {
  animalType: "cat";
  breedName: string;
  coloration: "tabby" | "solid-colored" | "spotted";
};
type Dog = {
  animalType: "dog";
  breedName: string;
  size: "teacup" | "toy" | "standard" | "huge";
};
type PetInfo = { name: string; familyName: string };

// We can define types that are both pet and Cat (or Dog) by intersecting
// the PetInfo type with the species type.
type PetCat = PetInfo & Cat;
type PetDog = PetInfo & Dog;

/*
 * ======================================================
 * TODO: Describe a valid PetCat and PetDog below.
 * HINT: Use autocompletion (ctrl-space) to help you fill in the properties.
 * ======================================================*/
const sukiTheCat: PetCat = {};
const finnTheDog: PetDog = {};

function announcePet(pet: PetInfo) {
  return `This is ${pet.familyName} family pet, ${pet.name}.`;
}

/*
 * Notice that announcePet will take any object that's
 * structurally compatible with type Pet. */
announcePet(sukiTheCat);
announcePet(finnTheDog);










/* ======================================================
 * TS understands control flow.
 * 
 * TODO: Take a look at the type of 'fruit' each time it's referenced in
 * isBanana below.
 * ====================================================== */
function isBanana(fruit: 'banana' | 'apple' | 'pear') {
  if (fruit === "banana") {
    return `${fruit} is a banana`;
  } else {
    return ` ${fruit} is not a banana`;
  }
}

/* ======================================================
 * TODO: Check out the return type of classify. TS has figured out
 * which specific strings can possibly be returned.
 * ====================================================== */
function classify(n: number) {
  if (n < 0) return "negative";
  if (n > 0) return "positive";
  return "zero";
}

/*
 * TS checks our declared return type against the inferred type
 * from all code paths in a function.
 * 
 * This makes it easy to tell if we've handled all cases.
 *
 * ======================================================
 * TODO: Try commenting out one of the return statements in
 *       describeNumber and see what happens.
 * ======================================================*/
function describeNumber(num: number): string {
  const value = classify(num);

  if (value === "negative") {
    return `${num} is a negative number`;
  } else if (value === "positive") {
    return `${num} is a positive number`;
  } else {
    return `${num} is zero`;
  }
}

  
/*
 * ======================================================
 * TODO: Change ONLY the input type FruitColor so that the
 * function below will only accept inputs it is able to classify
 * ======================================================*/
type FruitColor = string;

function appleOrBanana(fruitColor: FruitColor): "apple" | "banana" {
  switch (fruitColor) {
    case "red":
      return "apple";
    case "yellow":
      return "banana";
  }
}


/*
 * Let's take a look at the HotDrink type described in the slides.
 *
 * TS can narrow types as we move through control flow. When
 * we're handling different cases of a union type, it's helpful
 * to have a single property that is shared between all the cases,
 * but has a different literal value for each case in the union. This allows
 * us to use type narrowing to determine which case we're dealing with.
 *
 * This is great for when we have a bunch of similar objects with
 * different constraints
 * 
 * This technique - uninioning object types together with a shared, discriminating field - 
 * is called a "discriminated union"
 */
type Tea = {
  type: "tea"; // Discriminant field
  style: "green" | "black" | "herbal";
  name: string;
};
type Coffee = {
  type: "coffee"; // Discriminant field
  roast: "dark" | "medium" | "light";
  name: string;
};

type HotDrink = Tea | Coffee;

/**
 * Since type is common to both Tea and Coffee, we can test it to
 * figure out which we're dealing with.
 * 
 * ======================================================
 * TODO: Write a function that takes a HotDrink and
 * returns the style, for tea, or the roast, for coffee.
 * ======================================================
 */
function describe(
  drink: HotDrink
): "green" | "black" | "herbal" | "dark" | "medium" | "light" {}

const rachaelsDrink: Tea = {name: "Chamomile", style: "herbal", type: "tea"}
const drewsDrink: Coffee = {name: "Onyx Columbia San Jose", roast: "light", type: "coffee"}

const herbalStyle = describe(rachaelsDrink);
const lightStyle = describe(drewsDrink);



/*
 * ======================================================
 * TODO: Define FruitType as a discriminated union so that
 * the type proves that apples are red and can be polished,
 * and bananas are yellow and can be peeled.
 * ======================================================
 */

type FruitType = unknown;

// Hint: To define a function property within an object, write something like:
// type SomeObject = {
//    myFunc: () => void
// };

function doSomething(fruit: FruitType) {
  // FruitType must have a `type` field as a discriminant
  switch (fruit.type) {
    case "apple":
      // Apples must be the color red in this example
      const color: "red" =  fruit.color
      console.log(color);

      // For this example, apples MUST NOT be yellow
      // typings:expect-error
      const wrongColor: "yellow" =  fruit.color
      console.log(wrongColor);

      // Apples must have a polish function property
      fruit.polish();

      // Apples must not have a peel function property
      // typings:expect-error
      fruit.peel();

      break;

    case "banana":
      // Bananas must be yellow in our example
      const color: "yellow" =  fruit.color
      console.log(color);

      // Bananas must not be red.
      // typings:expect-error
      const wrongColor: "red" =  fruit.color
      console.log(wrongColor);

      // We can peel a banana
      fruit.peel();

      // But polishing a banana would be silly.
      // typings:expect-error
      fruit.polish();

      break;
  }
}