Exhaustive pattern matching in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is exhaustive pattern matching in TypeScript?

Exhaustive pattern matching means checking all possible cases of a value so that no case is missed. It helps catch errors when new cases are added.

intermediate

How can you enforce exhaustive checks in TypeScript's switch statements?

You can add a default case that throws an error or uses the never type to make sure all cases are handled.

intermediate

What is the never type used for in exhaustive pattern matching?

The never type represents values that should never happen. Using it in a default case helps TypeScript warn if some cases are missing.

beginner

Why is exhaustive pattern matching helpful in real-life programming?

It prevents bugs by making sure you handle every possible case, especially when your data types change or grow over time.

intermediate

Show a simple example of exhaustive pattern matching with a switch on a union type in TypeScript.

```typescript
 type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; size: number };
 function area(shape: Shape) {
   switch (shape.kind) {
     case 'circle':
       return Math.PI * shape.radius ** 2;
     case 'square':
       return shape.size ** 2;
     default:
       const _exhaustiveCheck: never = shape;
       return _exhaustiveCheck;
   }
 }
```

What does the never type represent in TypeScript?

A A number value

B A string value

C A value that never occurs

D Any possible value

The never type means a value that should never happen, useful for exhaustive checks.

Why add a default case with a never type in a switch?

A To catch missing cases and get TypeScript errors

B To ignore all cases

C To return a default value

D To make the code shorter

Adding a default with never helps TypeScript warn if some cases are not handled.

What happens if you forget a case in exhaustive pattern matching?

A The program always crashes

B The missing case is handled automatically

C Nothing happens, code runs fine

D TypeScript can warn you if set up correctly

If you use exhaustive checks, TypeScript warns you about missing cases to avoid bugs.

Which TypeScript feature helps you create union types for pattern matching?

A Union types using |

B Classes

C Interfaces only

D Enums only

Union types with | let you combine types for pattern matching.

What is the main benefit of exhaustive pattern matching?

A Avoid using switch statements

B Catch all possible cases and avoid bugs

C Reduce code size

D Make code run faster

Exhaustive matching ensures all cases are handled, preventing bugs.

Explain how to implement exhaustive pattern matching in TypeScript using a switch statement.

Think about how to make TypeScript check all cases.

Why is exhaustive pattern matching important when your data types change over time?

Consider what happens if you add a new case but forget to handle it.