Maxi Contieri

Posted on Nov 13, 2020 • Edited on Jul 5, 2021 • Originally published at maximilianocontieri.com

Code Smell 23 - Instance Type Checking

#oop #beginners #tutorial

Do you check who are you talking to?

TL;DR: Trust your collaborators. Don't check who they are. Ask them to do instead.

Problems

Coupling
Metamodel interference
IFs

Solutions

Avoid kind, isKindOf, instance, getClass(), typeOf, etc..
Don't use Reflection and Metaprogramming for Domain Objects.
Replace IFs with polymorphism.
Avoid checking for 'undefined'. Use complete objects, avoid nulls and setters, favor immutability and you will never have undefined and ifs.

Sample Code

Wrong

if (typeof(x) === 'undefined') {
    console.log('variable x is not defined');   
}

function isNumber(data) {
  return (typeof data === 'number');
}

function move(animal) {
  if (animal instanceof Rabbit) {
      animal.run()
  }
  if (animal instanceof Seagull) {
      animal.fly()
  } 
}

class Rabbit {
  run(){
    console.log("I'm running");
  }  
}

class Seagull {
  fly(){
    console.log("I'm flying");
  }  
}

let bunny = new Rabbit();
let livingstone = new Seagull();

move(bunny);
move(livingstone);

Right

/*Avoid these methods
if (typeof(x) === 'undefined') {
    console.log('variable x is not defined');   
}

function isNumber(data) {
  return (typeof data === 'number');
}
*/

class Animal {
} 

class Rabbit extends Animal {
  move(){
    console.log("I'm running");
  }  
}

class Seagull extends Animal {
  move(){
    console.log("I'm flying");
  }  
}

let bunny = new Rabbit();
let livingstone = new Seagull();

bunny.move();
livingstone.move();

Detection

Since type checking methods are well known it is very easy to set up a code policy checking the uses.

Conclusion

Testing for a class type couples the objects with accidental decisions and violates bijection since no such control exists on real world. It is a smell our models are not good enough.