Union vs intersection mental model in Typescript - Quick Revision & Key Differences

Recall & Review

beginner

What is a union type in TypeScript?

A union type means a value can be one of several types. It uses the | symbol. For example, string | number means the value can be a string or a number.

beginner

What is an intersection type in TypeScript?

An intersection type means a value must have all the types combined. It uses the & symbol. For example, {a: number} & {b: string} means the value has both properties a and b.

beginner

How can you imagine a union type in real life?

Think of a union type like a menu where you can choose one item from several options. For example, choosing a drink: coffee or tea or juice.

beginner

How can you imagine an intersection type in real life?

Think of an intersection type like a combo meal that includes all parts together. For example, a meal with a burger AND fries AND a drink.

intermediate

What happens if you try to assign a value that fits neither union nor intersection types?

For a union type, the value must match at least one type. For an intersection type, the value must satisfy all types. If it doesn't, TypeScript will show an error.

Which symbol represents a union type in TypeScript?

A *

B |

C +

D &

The union type uses the pipe symbol | to mean 'or'.

Which symbol represents an intersection type in TypeScript?

A +

B |

C &

D *

The intersection type uses the ampersand symbol & to mean 'and'.

If a variable is typed as string | number, which value is allowed?

A 42

B {name: 'Alice'}

C true

D [1, 2, 3]

The value 42 is a number, which fits the union type string | number.

If a variable is typed as {a: number} & {b: string}, which object is valid?

A {}

B {b: 'hello'}

C {a: 5}

D {a: 5, b: 'hello'}

The object must have both properties a and b to satisfy the intersection type.

Which mental model best describes a union type?

A A menu where you pick one item

B A combo meal with all items included

C A list of unrelated items

D A single fixed item

A union type is like choosing one item from a menu of options.

Explain the difference between union and intersection types using a simple real-life example.

Think about choosing one item versus having all items together.

Describe how TypeScript uses the symbols | and & in types and what they mean.

Focus on the meaning of 'or' and 'and' in types.

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