Exhaustiveness checking

⬅️ FEM TypeScript Fundamentals

Exhaustiveness checking

• Exhaustive” means “considering all possibilities”

function getName(hasName: User | Cat | Dog, names: string[]): true {
  switch (hasName.kind) {
    case 'user':
      names.push(hasName.userName);
      return true;
	// OR
	  //case 'user':
      //return hasName.userName;
    case 'cat':
      names.push(hasName.catName);
      return true;
    case 'dog':
      names.push(hasName.dogName);
      return true;
  }
}
 
const names: string[] = [];
getName({kind: 'user', userName: 'Amir'}, names);
getName({kind: 'cat', catName: 'Ms. Fluff'}, names);
getName({kind: 'dog', dogName: 'Woofy'}, names);
 
names;
 
//Result:
 
['Amir', 'Ms. Fluff', 'Woofy']

• The true value isn’t useful to us at runtime, but we have to return something in order to use exhaustiveness checking.
• Generally, we associate exhaustiveness checking with switch. However, it works with if as well.
• In TypeScript, providing a default: inside of our switch effectively disables exhaustiveness checking.

So in summary when using switch:

1. Put the switch inside of a function where each case returns.
2. Give the function an explicit return type.
3. Don’t add a default unless you’re very sure that you need one.