Union vs intersection mental model in Typescript - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of union type variable assignment

What will be the output of the following TypeScript code?

Typescript

type A = { name: string } | { age: number };

const person: A = { name: "Alice", age: 30 };
console.log(person);

A { name: "Alice", age: 30 }

B TypeScript error: Object literal may only specify known properties

C { name: "Alice" }

D { age: 30 }

💡 Hint

For union types of object shapes, object literals must match exactly one member without excess properties due to TypeScript's excess property checking.

The union type requires the object to match either { name: string } or { age: number } exactly. The literal has excess 'age' for the first and excess 'name' for the second, causing a TypeScript compilation error due to excess property checking.

❓ Predict Output

intermediate

Output of intersection type variable assignment

What will be the output of this TypeScript code?

Typescript

type B = { name: string } & { age: number };

const person: B = { name: "Bob", age: 25 };
console.log(person);

A { name: "Bob", age: 25 }

B TypeScript error: Type '{ name: string; age: number; }' is not assignable to type 'B'

C { name: "Bob" }

D { age: 25 }

💡 Hint

Intersection means the object must have all properties from both types.

The intersection type requires the object to have both name and age properties. The object literal satisfies this, so it prints the full object.

🧠 Conceptual

advanced

Understanding union vs intersection types

Which statement best describes the difference between union and intersection types in TypeScript?

A Union types allow a value to be any one of the types, while intersection types require a value to satisfy all types simultaneously.

B Union types and intersection types are the same and can be used interchangeably.

C Union types require a value to satisfy all types simultaneously, while intersection types allow a value to satisfy any one of the types.

D Union types only work with primitive types, while intersection types only work with objects.

💡 Hint

Think about whether the value needs to meet one or all conditions.

Union types mean the value can be one of several types. Intersection types mean the value must satisfy all types at once.

❓ Predict Output

advanced

Output of function with union and intersection parameters

What is the output of this TypeScript code?

Typescript

type X = { a: number } | { b: string };

type Y = { a: number } & { b: string };

function testUnion(x: X) {
  if ('a' in x) return x.a;
  return x.b.length;
}

function testIntersection(y: Y) {
  return y.a + y.b.length;
}

console.log(testUnion({ a: 10 }));
console.log(testUnion({ b: "hello" }));
console.log(testIntersection({ a: 5, b: "world" }));

A

TypeScript error
TypeScript error
TypeScript error

B

01
5
01

C

10
5
TypeScript error

D

10
5
10

💡 Hint

Check how the functions handle union and intersection types differently.

testUnion checks which property exists and returns accordingly. testIntersection requires both properties and sums them. The outputs are 10, 5, and 10 respectively.

🔧 Debug

expert

Why does this intersection type assignment fail?

Why does the following TypeScript code produce an error?

Typescript

type C = { x: number } & { y: number };

const obj: C = { x: 1 };

A Because intersection types cannot be assigned to variables.

B Because obj has an extra property 'x' not allowed in intersection types.

C Because obj is missing property 'y' required by the intersection type.

D Because TypeScript does not support intersection types with objects.

💡 Hint

Think about what intersection means for required properties.

Intersection types require all properties from all types. obj is missing 'y', so assignment fails.

Practice

1. Which statement best describes a union type in TypeScript?

easy

A. A value must have all combined types, like string & number.

B. A value can be one type or another, like string | number.

C. It restricts a value to only one specific type.

D. It creates a new type by excluding some properties.

Solution

1. Step 1: Understand union type meaning

   A union type means a value can be one of several types, for example, string | number means the value can be either a string or a number.

2. Step 2: Compare with intersection type

   An intersection type requires a value to have all combined types, which is different from union. So a value can be one type or another, like string | number, correctly describes union.

3. Final Answer:

   A value can be one type or another, like string | number. -> Option B

4. Quick Check:

   Union = one of multiple types [OK]

Hint: Union means one type OR another, intersection means ALL types [OK]

Common Mistakes:

• Confusing union with intersection
• Thinking union requires all types simultaneously
• Mixing up syntax symbols | and &

2. Which of the following is the correct syntax for an intersection type combining A and B?

easy

A. type C = A | B;

B. type C = A, B;

C. type C = A + B;

D. type C = A & B;

Solution

1. Step 1: Recall intersection syntax

   Intersection types use the ampersand & symbol to combine types, meaning the value must satisfy both types.

2. Step 2: Identify correct syntax

   type C = A & B; uses A & B, which is the correct syntax for intersection types in TypeScript.

3. Final Answer:

   type C = A & B; -> Option D

4. Quick Check:

   Intersection uses & symbol [OK]

Hint: Intersection uses & symbol, union uses | symbol [OK]

Common Mistakes:

• Using | instead of & for intersection
• Using invalid operators like + or comma
• Confusing type alias syntax

3. What is the type of variable value after this assignment?

type A = { name: string };
type B = { age: number };

let value: A | B = { name: "Alice", age: 30 };

medium

A. Error: value must be either A or B, not both.

B. Valid: value must have only name property.

C. Valid: value has both name and age properties.

D. Error: value must have only age property.

Solution

1. Step 1: Understand union type assignment

   The variable value is declared as A | B, meaning it can be either type A or type B.

2. Step 2: Check assigned object

   The assigned object has both name and age properties, which does not strictly satisfy either A or B alone, so it is not valid as a union type.

3. Final Answer:

   Error: value must be either A or B, not both. -> Option A

4. Quick Check:

   Union requires value to match one type exactly [OK]

Hint: Union requires value to match one type exactly [OK]

Common Mistakes:

• Thinking union allows objects with all properties combined
• Confusing union with intersection requirements
• Assuming extra properties are allowed in union

4. Identify the error in this TypeScript code:

type A = { name: string };
type B = { age: number };

let person: A & B = { name: "Bob" };

medium

A. Syntax error in type declaration.

B. Incorrect use of union instead of intersection.

C. Missing property age in the object assigned to person.

D. Extra property name not allowed.

Solution

1. Step 1: Understand intersection type requirements

   An intersection type A & B requires the value to have all properties from both types A and B.

2. Step 2: Check assigned object properties

   The object assigned has only name property but is missing age, so it does not satisfy the intersection type.

3. Final Answer:

   Missing property age in the object assigned to person. -> Option C

4. Quick Check:

   Intersection requires all properties present [OK]

Hint: Intersection needs all properties present [OK]

Common Mistakes:

• Thinking intersection allows partial properties
• Confusing union and intersection requirements
• Ignoring missing properties error

5. Given these types:

type Name = { name: string };
type Age = { age: number };
type Contact = { email: string };

// Which type correctly represents a value that must have a name and either age or email?

// Choose the correct type for Person:

hard

A. type Person = Name & (Age | Contact);

B. type Person = (Name & Age) | Contact;

C. type Person = Name | Age & Contact;

D. type Person = Name & Age & Contact;

Solution

1. Step 1: Analyze requirement

   The value must have a name property and either an age or email property.

2. Step 2: Translate to types

   This means the type must be Name AND (either Age OR Contact), which is Name & (Age | Contact).

3. Step 3: Check options

   type Person = Name & (Age | Contact); matches this exactly. Other options either change the logic or require all three properties.

4. Final Answer:

   type Person = Name & (Age | Contact); -> Option A

5. Quick Check:

   Name AND (Age OR Contact) = Name & (Age | Contact) [OK]

Hint: Use parentheses to combine union and intersection correctly [OK]

Common Mistakes:

• Ignoring parentheses changing logic
• Mixing order of union and intersection
• Choosing all properties instead of either/or