Discriminated unions in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a discriminated union type for shapes.

Typescript

type Shape = Circle | Square;

interface Circle {
  kind: [1];
  radius: number;
}

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

A"circle"

B"radius"

C"square"

D"kind"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the property name instead of the string literal value.

Forgetting to use quotes around the string literal.

The kind property is used as the discriminant with the value "circle" to identify the Circle type.

2fill in blank

medium

Complete the code to add a Square type to the discriminated union.

Typescript

interface Square {
  kind: [1];
  sideLength: number;
}

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

A"sideLength"

B"circle"

C"square"

D"kind"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong string literal for the kind property.

Confusing property names with discriminant values.

The kind property for Square should be the string literal "square" to distinguish it from other shapes.

3fill in blank

hard

Fix the error in the function to correctly narrow the shape type using the discriminated union.

Typescript

function area(shape: Shape): number {
  if (shape.kind === [1]) {
    return Math.PI * shape.radius ** 2;
  } else {
    return shape.sideLength * shape.sideLength;
  }
}

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

A"radius"

B"circle"

C"square"

D"sideLength"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for the wrong discriminant value.

Using property names instead of discriminant string literals.

The discriminant check should compare shape.kind to "circle" to narrow the type to Circle.

4fill in blank

hard

Fill both blanks to create a discriminated union and a function that returns the perimeter.

Typescript

type Shape = [1] | [2];

function perimeter(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return 2 * Math.PI * shape.radius;
    case "square":
      return 4 * shape.sideLength;
  }
}

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

ACircle

BSquare

CRectangle

DTriangle

Attempts:

3 left

💡 Hint

Common Mistakes

Using types not defined in the union.

Mixing unrelated shape types.

The discriminated union Shape is made of Circle and Square types.

5fill in blank

hard

Fill all three blanks to define a discriminated union with a Triangle type and extend the area function.

Typescript

interface Triangle {
  kind: [1];
  base: number;
  height: number;
}

type Shape = Circle | Square | [2];

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength * shape.sideLength;
    case [3]:
      return 0.5 * shape.base * shape.height;
  }
}

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

A"triangle"

BTriangle

DSquare

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching discriminant strings and case labels.

Forgetting to add Triangle to the union type.

The Triangle interface uses the discriminant "triangle". The union includes Triangle, and the switch case uses the string "triangle" to calculate the area.

Practice

1.

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

easy

A. To automatically convert types at runtime

B. To create a union of unrelated primitive types

C. To enforce a single type for all variables

D. To distinguish between different object types using a common property

Solution

1. Step 1: Understand discriminated unions

   Discriminated unions use a shared property (called the discriminant) to identify which type an object belongs to.

2. Step 2: Purpose of discriminated unions

   This helps TypeScript safely narrow types in code by checking that property.

3. Final Answer:

   To distinguish between different object types using a common property -> Option D

4. Quick Check:

   Discriminated union = common property to tell types apart [OK]

Hint: Look for a shared property that identifies each type [OK]

Common Mistakes:

• Thinking discriminated unions convert types automatically
• Confusing with simple unions of primitives
• Believing they enforce one single type only

2.

Which of the following is the correct syntax to define a discriminated union type in TypeScript?

type Shape = ?

easy

A. type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number }

B. type Shape = { radius: number } & { side: number }

C. type Shape = { kind: string; radius?: number; side?: number }

D. type Shape = { circle: number } | { square: number }

Solution

1. Step 1: Identify discriminant property

   type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number } uses a common property kind with literal types ('circle' or 'square') to discriminate.

2. Step 2: Check union structure

   type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number } correctly uses a union of two object types distinguished by kind.

3. Final Answer:

   type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number } -> Option A

4. Quick Check:

   Discriminated union = union with common literal property [OK]

Hint: Look for union with a shared literal property for discrimination [OK]

Common Mistakes:

• Using intersection (&) instead of union (|)
• Missing literal type for discriminant property
• Using optional properties without discriminant

3.

What will be the output of the following TypeScript code?

type Animal =
  | { type: 'dog'; barkVolume: number }
  | { type: 'cat'; livesLeft: number };

function describe(animal: Animal) {
  switch (animal.type) {
    case 'dog':
      return `Dog barks at volume ${animal.barkVolume}`;
    case 'cat':
      return `Cat has ${animal.livesLeft} lives left`;
  }
}

console.log(describe({ type: 'cat', livesLeft: 7 }));

medium

A. Cat has 7 lives left

B. Dog barks at volume undefined

C. TypeError at runtime

D. Cat has undefined lives left

Solution

1. Step 1: Analyze the input object

   The input is { type: 'cat', livesLeft: 7 }, so the type is 'cat'.

2. Step 2: Follow the switch case

   The switch matches 'cat' case and returns `Cat has ${animal.livesLeft} lives left`, which becomes 'Cat has 7 lives left'.

3. Final Answer:

   Cat has 7 lives left -> Option A

4. Quick Check:

   Switch on discriminant returns correct string [OK]

Hint: Match the discriminant property in switch to get output [OK]

Common Mistakes:

• Confusing 'dog' and 'cat' cases
• Expecting runtime error due to union
• Ignoring the discriminant property

4.

Identify the error in this TypeScript code using discriminated unions:

type Vehicle =
  | { kind: 'car'; wheels: number }
  | { kind: 'bike'; pedals: number };

function getWheels(vehicle: Vehicle) {
  if (vehicle.kind === 'car') {
    return vehicle.wheels;
  } else {
    return vehicle.wheels;
  }
}

medium

A. Function should use switch instead of if

B. Property 'wheels' does not exist on type 'bike' object

C. Incorrect discriminant property name

D. Missing return type annotation on function

Solution

1. Step 1: Check property access in else branch

   In the else branch, the code accesses vehicle.wheels, but bike type has no wheels property.

2. Step 2: Understand TypeScript error

   This causes a type error because wheels is not defined for { kind: 'bike'; pedals: number }.

3. Final Answer:

   Property 'wheels' does not exist on type 'bike' object -> Option B

4. Quick Check:

   Accessing wrong property on union member causes error [OK]

Hint: Check property exists on all union members before access [OK]

Common Mistakes:

• Assuming all union members share same properties
• Ignoring TypeScript error messages
• Thinking switch is mandatory over if

5.

Given the following discriminated union, how can you safely add a new shape type rectangle with width and height properties and update the area function accordingly?

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; side: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2;
    case 'square':
      return shape.side ** 2;
  }
}

hard

A. Add { kind: 'rectangle'; width: number; height: number } to Shape union and replace switch with if-else

B. Add { kind: 'rectangle'; width: number; height: number } to Shape union but no change needed in area function

C. Add { kind: 'rectangle'; width: number; height: number } to Shape union and add case 'rectangle' in switch returning width * height

D. Add { kind: 'rectangle'; width: number; height: number } to Shape union and remove existing cases

Solution

1. Step 1: Extend the union type

   Add the new shape type with discriminant kind: 'rectangle' and properties width and height to the Shape union.

2. Step 2: Update the area function

   Add a new case in the switch for 'rectangle' that returns shape.width * shape.height to correctly calculate the area.

3. Final Answer:

   Add { kind: 'rectangle'; width: number; height: number } to Shape union and add case 'rectangle' in switch returning width * height -> Option C

4. Quick Check:

   Extend union and handle new case in switch [OK]

Hint: Add new type to union and handle it in switch [OK]

Common Mistakes:

• Forgetting to update the function for new type
• Removing existing cases accidentally
• Not using discriminant property in new type