simple decision logic demo for a friend's son currently javascripting

can_pet.js

// more javascripty!

const FRIENDLY = {
  MAYBE: 'pet at your own risk',
  YES: 'ok to pet',
  NO: 'do NOT pet'
}

class Pet {
  constructor(name, isFriendly = FRIENDLY.MAYBE, sound = "UNKNOWN") {
    this.name = name;
    this.isFriendly = isFriendly;
    this.sound = sound;
  }
  soundsLike() { return this.sound; }
  canPet() { return this.isFriendly; }
}

class Dog extends Pet {
  constructor(name) { super(name, FRIENDLY.YES, "woof!"); }
}

class Cat extends Pet {
  constructor(name) { super(name, FRIENDLY.MAYBE, "meow!"); }
}

dog = new Dog("Spot");
cat = new Cat("Whisper");
what = new Pet("MysteryMeat");

// works but there are better ways
// deliberately ignoring the class functions here
const canPetIf = (pet) => {
  let klass = pet.constructor.name;
  console.log("---");
  if (klass === 'Dog') {
    console.log(`can you pet it? ${FRIENDLY.YES}.`);
  } else if (klass === 'Cat') {
    console.log(`can you pet it? ${FRIENDLY.MAYBE}.`);
  } else if (klass === 'Pet') {
    console.log(`can you pet it? ${FRIENDLY.MAYBE}.`);
  } else if (pet instanceof Pet) {
    console.log(`can you pet it? ${FRIENDLY.NO}.`);
  } else {
    // bounce if it's not a Pet
    console.log(`${pet} doesn't look like a pet to me.`);
    return;
  }
  console.log(`this critter is a ${klass} named ${pet.name}`);
  console.log(`they make a '${pet.soundsLike()}' noise`);
};

console.log("\n### using `if/then/else`:");
canPetIf(dog);
canPetIf(cat);
canPetIf(what);
canPetIf('paintbrush');

// better, especially with many optionss
const canPetSwitch = (pet) => {
  let klass = pet.constructor.name;
  console.log("---");
  switch(klass) {
    case 'Dog': {
      console.log(`can you pet it? ${FRIENDLY.YES}.`);
      break;
    }
    case 'Cat': {
      console.log(`can you pet it? ${FRIENDLY.MAYBE}.`);
      break;
    }
    case 'Pet': {
      console.log(`can you pet it? ${FRIENDLY.MAYBE}.`);
      break;
    }
    default: {
      // we can catch previously undefined types if we want
      if (pet instanceof Pet) {
        console.log(`can you pet it? ${FRIENDLY.NO}.`);
      } else {
        // bounce if it's not a Pet
        console.log(`${pet} doesn't look like a pet to me.`);
        return;
      }
    }
  }
  console.log(`this critter is a ${klass} named ${pet.name}`);
  console.log(`they make a '${pet.soundsLike()}' noise`);
}

console.log("\n\n### using `switch`:");
canPetSwitch(dog);
canPetSwitch(cat);
canPetSwitch(what);
canPetSwitch("ham sandwich");

// using types and class functions, it doesn't matter when we define the new
// type. the function will still behave.
class VampireBat extends Pet {
  constructor(name) { super(name, FRIENDLY.NO, "HIGH PITCH SQUEAKING"); }
}

bat = new VampireBat("Dracula");
canPetSwitch(bat);

// the best way in an OO language is to make all the things you
// care about accessible regardless of type. no decision logic at all.
const canPetOO = (pet) => {
  console.log("---");
  // guard clause:
  if (!(pet instanceof Pet)) {
    console.log(`${pet} doesn't look like a pet to me.`);
    return; // you only get to use the Pet functions if it's a Pet instance
  }

  console.log(`this critter is a ${ pet.constructor.name } is named ${ pet.name }`);
  console.log(`they make a '${pet.soundsLike()}' noise`);
  console.log(`can you pet it? ${ pet.canPet() }.`);
}

console.log("\n\n### using type inheritance only:");
canPetOO(dog);
canPetOO(cat);
canPetOO(what);
canPetOO(bat);
canPetOO(Math.PI);

output.txt

### using `if/then/else`:
---
can you pet it? ok to pet.
this critter is a Dog named Spot
they make a 'woof!' noise
---
can you pet it? pet at your own risk.
this critter is a Cat named Whisper
they make a 'meow!' noise
---
can you pet it? pet at your own risk.
this critter is a Pet named MysteryMeat
they make a 'UNKNOWN' noise
---
paintbrush doesn't look like a pet to me.


### using `switch`:
---
can you pet it? ok to pet.
this critter is a Dog named Spot
they make a 'woof!' noise
---
can you pet it? pet at your own risk.
this critter is a Cat named Whisper
they make a 'meow!' noise
---
can you pet it? pet at your own risk.
this critter is a Pet named MysteryMeat
they make a 'UNKNOWN' noise
---
ham sandwich doesn't look like a pet to me.
---
can you pet it? do NOT pet.
this critter is a VampireBat named Dracula
they make a 'HIGH PITCH SQUEAKING' noise


### using type inheritance only:
---
this critter is a Dog is named Spot
they make a 'woof!' noise
can you pet it? ok to pet.
---
this critter is a Cat is named Whisper
they make a 'meow!' noise
can you pet it? pet at your own risk.
---
this critter is a Pet is named MysteryMeat
they make a 'UNKNOWN' noise
can you pet it? pet at your own risk.
---
this critter is a VampireBat is named Dracula
they make a 'HIGH PITCH SQUEAKING' noise
can you pet it? do NOT pet.
---
3.141592653589793 doesn't look like a pet to me.