Discriminated union state machines in Typescript

Introduction

Discriminated union state machines help you manage different states clearly and safely in your program. They make it easy to know what state you are in and what actions you can do next.

When you want to track different steps in a process, like a login flow.

When you need to handle different modes of a feature, like a media player (playing, paused, stopped).

When you want to avoid mistakes by making sure only valid states and transitions happen.

When you want your code to be easier to read and maintain by clearly defining states.

When you want TypeScript to help catch errors about states before running the program.

Syntax

Typescript

type State =
  | { type: 'idle' }
  | { type: 'loading' }
  | { type: 'success'; data: string }
  | { type: 'error'; message: string };

function handleState(state: State) {
  switch (state.type) {
    case 'idle':
      // handle idle
      break;
    case 'loading':
      // handle loading
      break;
    case 'success':
      // handle success with state.data
      break;
    case 'error':
      // handle error with state.message
      break;
  }
}

The type property is the discriminant that tells TypeScript which state it is.

Each state can have its own extra properties to hold data related to that state.

Examples

This defines a traffic light state machine with three states.

Typescript

type LightState =
  | { type: 'red' }
  | { type: 'yellow' }
  | { type: 'green' };

This example shows a login state machine with a user name in the loggedIn state.

Typescript

type AuthState =
  | { type: 'loggedOut' }
  | { type: 'loggingIn' }
  | { type: 'loggedIn'; user: string };

function printState(state: AuthState) {
  switch (state.type) {
    case 'loggedOut':
      console.log('User is logged out');
      break;
    case 'loggingIn':
      console.log('User is logging in');
      break;
    case 'loggedIn':
      console.log(`Welcome, ${state.user}`);
      break;
  }
}

Sample Program

This program models a door that can be closed, opening, open, or closing. It moves through states in order and prints each state.

Typescript

type DoorState =
  | { type: 'closed' }
  | { type: 'opening' }
  | { type: 'open' }
  | { type: 'closing' };

function nextState(state: DoorState): DoorState {
  switch (state.type) {
    case 'closed':
      return { type: 'opening' };
    case 'opening':
      return { type: 'open' };
    case 'open':
      return { type: 'closing' };
    case 'closing':
      return { type: 'closed' };
  }
}

let state: DoorState = { type: 'closed' };
console.log(`Initial state: ${state.type}`);

state = nextState(state);
console.log(`Next state: ${state.type}`);

state = nextState(state);
console.log(`Next state: ${state.type}`);

state = nextState(state);
console.log(`Next state: ${state.type}`);

state = nextState(state);
console.log(`Next state: ${state.type}`);

Important Notes

Always use a unique type string for each state to help TypeScript distinguish them.

Using a switch statement on the discriminant helps TypeScript know which properties are available.

This pattern helps prevent bugs by making invalid states or transitions impossible to represent.

Summary

Discriminated unions let you define clear, safe states with a common type property.

Use them to build simple state machines that are easy to understand and maintain.

TypeScript helps catch mistakes by knowing exactly which state you are working with.

Practice

1.

What is the main purpose of using a discriminated union in TypeScript state machines?

easy

A. To enable dynamic addition of states at runtime without type errors.

B. To clearly define different states with a common type property for safe type checking.

C. To allow any property names in the state objects without restrictions.

D. To automatically generate UI components based on state names.

Solution

1. Step 1: Understand discriminated unions

   Discriminated unions use a common type property to distinguish between different object shapes.

2. Step 2: Identify the purpose in state machines

   This common property allows TypeScript to safely narrow the state type and catch errors.

3. Final Answer:

   To clearly define different states with a common type property for safe type checking. -> Option B

4. Quick Check:

   Discriminated union = safe state typing [OK]

Hint: Look for the common type property in states [OK]

Common Mistakes:

• Thinking discriminated unions allow any property names freely
• Assuming states can be added dynamically without type errors
• Confusing discriminated unions with UI generation

2.

Which of the following is the correct syntax to define a discriminated union type for a state machine with states loading and success?

type State = ?

easy

A. { status: 'loading' } | { status: 'success'; data: string }

B. { type: 'loading'; data: string } | { type: 'success' }

C. { type: 'loading' } | { type: 'success'; data: string }

D. { state: 'loading' } | { state: 'success'; data: string }

Solution

1. Step 1: Identify the discriminant property

   The discriminated union requires a common property named type to distinguish states.

2. Step 2: Match the correct state shapes

   Loading state has only type: 'loading', success state has type: 'success' and extra data.

3. Final Answer:

   { type: 'loading' } | { type: 'success'; data: string } -> Option C

4. Quick Check:

   Common type property with correct fields = B [OK]

Hint: Discriminant property must be named exactly 'type' [OK]

Common Mistakes:

• Using different property names like 'status' or 'state'
• Mixing properties between states incorrectly
• Adding data to loading state incorrectly

3.

Given the following TypeScript code, what will be the output?

type State =
  | { type: 'idle' }
  | { type: 'loading' }
  | { type: 'success'; data: string };

function getMessage(state: State): string {
  switch (state.type) {
    case 'idle':
      return 'Waiting...';
    case 'loading':
      return 'Loading...';
    case 'success':
      return `Data: ${state.data}`;
  }
}

console.log(getMessage({ type: 'success', data: 'Hello' }));

medium

A. Error: Property 'data' does not exist

B. Loading...

C. Waiting...

D. Data: Hello

Solution

1. Step 1: Analyze the input state

   The input state is { type: 'success', data: 'Hello' }, matching the 'success' case.

2. Step 2: Follow the switch case logic

   For 'success', the function returns `Data: ${state.data}`, which becomes 'Data: Hello'.

3. Final Answer:

   Data: Hello -> Option D

4. Quick Check:

   State 'success' returns data string [OK]

Hint: Match type in switch to get correct output [OK]

Common Mistakes:

• Confusing 'success' with 'loading' or 'idle'
• Expecting an error accessing 'data' without checking type
• Ignoring template string interpolation

4.

Identify the error in this discriminated union state machine code:

type State =
  | { type: 'start' }
  | { type: 'end'; result: number };

function process(state: State) {
  if (state.type === 'start') {
    console.log('Starting');
  } else {
    console.log('Result is ' + state.result);
  }
}

process({ type: 'start' });

medium

A. No error; code runs and prints 'Starting'.

B. Error: Accessing 'result' on 'start' state causes runtime error.

C. Error: Missing return type annotation on function.

D. Error: 'type' property should be optional.

Solution

1. Step 1: Check the state type conditions

   The function checks if state.type === 'start' and logs 'Starting'.

2. Step 2: Confirm safe access to 'result'

   In the else branch, state.result is accessed only if state.type !== 'start', so it must be 'end' with result.

3. Final Answer:

   No error; code runs and prints 'Starting'. -> Option A

4. Quick Check:

   Type narrowing prevents invalid property access [OK]

Hint: Check type guards before accessing state-specific properties [OK]

Common Mistakes:

• Assuming accessing 'result' on 'start' state causes error
• Expecting return type annotation is mandatory
• Thinking 'type' property should be optional

5.

You want to extend a discriminated union state machine to handle an error state with an optional message. Which is the best way to define this state to keep type safety and allow missing message?

type State =
  | { type: 'idle' }
  | { type: 'loading' }
  | { type: 'success'; data: string }
  | ???

hard

A. { type: 'error'; message?: string }

B. { type: 'error'; message: string | undefined }

C. { type: 'error'; message: string }

D. { type: 'error'; message: string | null }

Solution

1. Step 1: Understand optional properties in discriminated unions

   Using message?: string means the property can be missing or a string, which fits optional message.

2. Step 2: Compare other options

   { type: 'error'; message: string | undefined } requires the property but allows undefined explicitly, less idiomatic. { type: 'error'; message: string } requires message always. { type: 'error'; message: string | null } requires message but allows null, which is less type safe.

3. Final Answer:

   { type: 'error'; message?: string } -> Option A

4. Quick Check:

   Optional property syntax keeps type safety and flexibility [OK]

Hint: Use '?' to mark optional properties in state types [OK]

Common Mistakes:

• Making message required when it should be optional
• Using union with undefined instead of optional property
• Allowing null which can cause runtime issues