Discriminated union state machines in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a discriminated union in TypeScript?

A discriminated union is a special type of union where each member has a common property with a unique literal type. This property helps TypeScript narrow down the type safely.

beginner

Why use discriminated unions for state machines?

They let you clearly define each state with a unique 'tag' property, making it easy to check the current state and handle transitions safely with TypeScript's type checking.

intermediate

Example of a discriminated union for a traffic light state machine?

type TrafficLight = | { state: 'red'; duration: number } | { state: 'yellow'; duration: number } | { state: 'green'; duration: number }; Each object has a 'state' property that discriminates the union.

intermediate

How does TypeScript help when switching on a discriminated union?

TypeScript narrows the type inside each case block based on the discriminant property, so you get autocompletion and type safety for that specific state.

advanced

What happens if you forget to handle a state in a discriminated union switch?

TypeScript can warn you if you use the 'never' type in a default case, helping you catch missing cases and making your state machine safer.

What property is essential in a discriminated union to distinguish states?

A A numeric index

B A method to change states

C A common property with unique literal values

D An optional property

The discriminant property is a common property with unique literal values that helps TypeScript identify the current type.

In a state machine using discriminated unions, how do you typically check the current state?

A Using if statements on random properties

B By using a global variable

C By checking the object's prototype

D Using a switch statement on the discriminant property

A switch on the discriminant property lets TypeScript narrow the type safely for each state.

What benefit does TypeScript provide when handling all states in a discriminated union?

A It ensures all cases are handled, preventing runtime errors

B It automatically fixes bugs

C It runs the code faster

D It hides unused states

TypeScript can warn about missing cases, helping prevent bugs from unhandled states.

Which of these is a valid discriminated union type for a simple state machine?

A type State = any

B type State = { status: 'on' } | { status: 'off' }

C type State = { value: number } & { value: string }

D type State = number | string

Option B uses a common property 'status' with unique literal types, which is a discriminated union.

What TypeScript feature helps catch missing cases in a discriminated union switch?

A The 'never' type in a default case

B The 'any' type

C The 'unknown' type

D The 'void' type

Using 'never' in a default case forces TypeScript to check exhaustiveness of the switch.

Explain how discriminated unions help build safe state machines in TypeScript.

Think about how TypeScript knows which state you are working with.

Describe how you would implement a simple traffic light state machine using discriminated unions.

Focus on the 'state' property as the discriminator.