Discriminated union state machines in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a discriminated union type for a state machine with states 'idle' and 'loading'.

Typescript

type State = { status: 'idle' } | { status: [1] };

Drag options to blanks, or click blank then click option'

A'done'

B'loading'

C'error'

D'start'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a state name not defined in the union, like 'done' or 'error'.

The discriminated union uses the 'status' property to distinguish states. Here, 'loading' is the correct second state.

2fill in blank

medium

Complete the code to create a function that returns true if the state is 'loading'.

Typescript

function isLoading(state: State): boolean {
  return state.status === [1];
}

Drag options to blanks, or click blank then click option'

A'loading'

B'idle'

C'done'

D'error'

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing to 'idle' or other incorrect states.

The function checks if the state's status is 'loading' to return true.

3fill in blank

hard

Fix the error in the switch statement to handle all states correctly.

Typescript

function handleState(state: State) {
  switch (state.status) {
    case 'idle':
      return 'Waiting';
    case [1]:
      return 'Loading data';
    default:
      return 'Unknown';
  }
}

Drag options to blanks, or click blank then click option'

A'done'

B'error'

C'loading'

D'start'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a case label that doesn't match any state, causing the default case to run.

The switch must handle the 'loading' state to return the correct message.

4fill in blank

hard

Fill both blanks to define a state machine with 'success' and 'failure' states using discriminated unions.

Typescript

type ResultState = { status: [1]; data: string } | { status: [2]; error: string };

Drag options to blanks, or click blank then click option'

A'success'

B'loading'

C'failure'

D'idle'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing unrelated states like 'loading' or 'idle' here.

The discriminated union defines 'success' with data and 'failure' with error.

5fill in blank

hard

Fill all three blanks to create a function that returns a message based on the ResultState.

Typescript

function getMessage(state: ResultState): string {
  switch (state.status) {
    case [1]:
      return `Data: ${state.[2]`;
    case [3]:
      return `Error: ${state.error}`;
  }
}

Drag options to blanks, or click blank then click option'

A'success'

Bdata

C'failure'

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property names or case labels that don't match the union.

The function checks if the state is 'success' or 'failure' and returns the appropriate message using the correct property.

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