Union vs intersection mental model in Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable that can be a string or a number using a union type.

Typescript

let value: string [1] number;

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

A/

B&

C|

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of | for union types

Using + or / which are not type operators

The | symbol is used to create a union type, meaning the variable can be either type.

2fill in blank

medium

Complete the code to declare a type that combines two interfaces using an intersection.

Typescript

type Person = Name [1] Age;

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

A&

B+

C|

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using | instead of & for intersection types

Using arithmetic operators like + or -

The & symbol creates an intersection type, combining all properties from both types.

3fill in blank

hard

Fix the error in the function parameter type to accept either a string or number using union.

Typescript

function printValue(value: string [1] number) { console.log(value); }

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

A&

B|

C,

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using & which means intersection, not union

Using commas or slashes which are invalid here

The function parameter should use a union type with | to accept string or number.

4fill in blank

hard

Fill both blanks to create a type that is either a string or a number and also has a length property.

Typescript

type StringOrNumberWithLength = (string [1] number) [2] { length: number };

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

A|

B&

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up union and intersection operators

Using arithmetic operators instead of type operators

First, use | to create a union of string or number, then use & to intersect with an object having a length property.

5fill in blank

hard

Fill all three blanks to create a function that accepts either a string or number and returns an object with the value and its type.

Typescript

function describeValue(value: string [1] number): { val: [2], type: string } { return { val: value, type: typeof value }; }

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

A&

Bstring | number

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of | for union types

Not matching the return type val with the parameter type

The parameter uses union | to accept string or number. The return type's val property is also string | number.

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