TypeScript Abstract Classes and Access Modifiers Quiz
A 40-question TypeScript quiz covering abstract classes, abstract methods, public/private/protected access, readonly properties, inheritance behavior, and visibility constraints.
40 Questions
~80 minutes
1
Question 1
What is an abstract class?
A
A class that cannot be instantiated directly
B
A class that must have no methods
C
A class that automatically freezes instances
D
A class used only for static fields
2
Question 2
What is true about abstract methods?
A
They must be implemented in subclasses
B
They can contain method bodies
C
They can only appear in interfaces
D
They are automatically optional
3
Question 3
Abstract classes can contain:
A
Both abstract and non-abstract methods
B
Only abstract methods
C
Only static methods
D
Only private fields
4
Question 4
Which access modifier makes a member visible everywhere?
A
public
B
protected
C
private
D
internal
5
Question 5
A private member is accessible to:
A
Only its own class
B
Its class and subclasses
C
Any class in the same file
D
Any caller using object reference
6
Question 6
A protected member is accessible by:
A
The class itself and its subclasses
B
All external callers
C
Only functions inside the same namespace
D
Only static methods
7
Question 7
What is wrong with this code?
typescript
abstract class Shape {}
const s = new Shape();A
Abstract classes cannot be instantiated
B
Abstract classes cannot be declared
C
Shape must extend another class
D
Abstract classes cannot contain fields
8
Question 8
What must a subclass do here?
typescript
abstract class Base {
abstract move(): void;
}A
Implement move()
B
Declare Base again
C
Make move() public
D
Remove abstract keyword
9
Question 9
What is the visibility of x?
typescript
class A {
protected x = 10;
}A
Visible only to A and its subclasses
B
Visible everywhere
C
Visible to the same file only
D
Visible only through static methods
10
Question 10
readonly fields:
A
Can only be assigned during declaration or in the constructor
B
Cannot be read at runtime
C
Cannot exist inside classes
D
Must always be public
11
Question 11
readonly works with which modifiers?
A
public, protected, and private
B
Only public
C
Only protected
D
Only private
12
Question 12
Subclasses of abstract classes must:
A
Implement all abstract members
B
Override all methods
C
Use only public methods
D
Declare their own abstract fields
13
Question 13
Which is true about overriding access modifiers?
A
You cannot make visibility more restrictive when overriding
B
You cannot make visibility more open when overriding
C
You must keep visibility the same always
D
Visibility is ignored during overriding
14
Question 14
Why is this valid?
typescript
abstract class A {
abstract draw(): void;
}
abstract class B extends A {}A
Because B is abstract and does not need to implement draw()
B
Because A has no constructor
C
Because draw() is private
D
Because abstract classes cannot extend others
15
Question 15
What is the visibility of run()?
typescript
class A {
private run() {}
}A
Accessible only within class A
B
Accessible in subclasses
C
Accessible everywhere
D
Accessible to objects of type A
16
Question 16
What can access this field?
typescript
class A {
protected value = 5;
}A
A and its subclasses
B
External callers only
C
Only static methods
D
Any file that imports A
17
Question 17
Abstract classes may contain:
A
Constructors
B
No constructors ever
C
Only private fields
D
Only static methods
18
Question 18
Can abstract properties exist?
A
Yes, subclasses must implement them
B
No, only methods can be abstract
C
Yes, but only if static
D
Yes, but only if private
19
Question 19
Access modifiers can be applied to:
A
Fields, methods, accessors, and constructors
B
Only fields
C
Only constructors
D
Only static members
20
Question 20
Why does this error?
typescript
abstract class A {
abstract x: number;
}
class B extends A {}A
Subclass must implement x
B
Abstract classes cannot have fields
C
x cannot be a number
D
B must also be abstract
21
Question 21
Why is this valid?
typescript
abstract class A {
abstract act(): void;
}
abstract class B extends A {
abstract step(): void;
}A
Abstract subclasses may defer abstract methods
B
act() is private
C
step() is optional
D
B overrides act()
22
Question 22
Which visibility is most restrictive?
A
private
B
protected
C
public
D
readonly
23
Question 23
public members are accessible:
A
From any location
B
From subclasses only
C
From same file only
D
From static contexts only
24
Question 24
protected prevents access from:
A
External callers outside the class hierarchy
B
Subclasses
C
Constructor functions
D
Methods inside the class
25
Question 25
private prevents access from:
A
Subclasses and external callers
B
Only external callers
C
Only constructors
D
Only abstract methods
26
Question 26
Why is this invalid?
typescript
class A {
protected x = 3;
}
const a = new A();
console.log(a.x);A
x is not visible outside the class hierarchy
B
protected cannot be used on fields
C
x must be private
D
protected fields cannot be logged
27
Question 27
Why does this error?
typescript
class A {
private run() {}
}
class B extends A {
run() {}
}A
private methods cannot be overridden
B
run() must be static
C
B must be abstract
D
A must be abstract
28
Question 28
Why is this valid?
typescript
abstract class A {
protected abstract step(): void;
}
class B extends A {
step() {}
}A
Subclasses must implement protected abstract members using compatible visibility
B
step() becomes public automatically
C
Abstract methods must become private
D
Abstract methods cannot be protected
29
Question 29
Abstract classes help:
A
Define shared templates with enforced subclass behavior
B
Prevent inheritance
C
Disable visibility rules
D
Replace all interfaces
30
Question 30
Access modifiers help:
A
Control which parts of a class can be accessed
B
Determine runtime performance
C
Auto-generate code
D
Force static-only usage
31
Question 31
readonly helps ensure:
A
Values do not change after construction
B
Fields cannot be read
C
Only private fields exist
D
Values must be numbers
32
Question 32
protected is useful when:
A
You want subclasses to access members without exposing them publicly
B
You want the strictest visibility
C
You need to lock all access
D
You want members available everywhere
33
Question 33
private is helpful when:
A
You want to enforce full encapsulation inside a class
B
You want subclass access
C
You need public inheritance
D
You want fields to be readonly
34
Question 34
Abstract classes differ from interfaces because:
A
They can contain actual implementation
B
They cannot contain methods
C
They cannot be extended
D
They cannot contain fields
35
Question 35
Subclasses may override methods with:
A
Equal or more accessible visibility
B
Less accessible visibility
C
Only private visibility
D
Any visibility ignoring the base class
36
Question 36
Abstract methods must:
A
Have no implementation in the base class
B
Always be static
C
Be private
D
Return void only
37
Question 37
public allows:
A
Full access with no restrictions
B
Access from subclasses only
C
Access from same file only
D
No access outside method bodies
38
Question 38
private prevents:
A
Subclass access completely
B
Access within same class
C
Access from other modules only
D
Access from static contexts
39
Question 39
Which scenario best fits abstract classes?
A
Sharing base behavior while requiring subclasses to complete details
B
Modeling shapes only through interfaces
C
When no shared behavior is needed
D
When constructors must be private
40
Question 40
Overall, access modifiers support:
A
Encapsulation and safe class design
B
Runtime mutation of visibility
C
Removing type checking
D
Overriding constructors only