Static members with types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing static member

What is the output of this TypeScript code?

Typescript

class Counter {
  static count: number = 0;
  increment() {
    Counter.count++;
  }
}

const c1 = new Counter();
c1.increment();
const c2 = new Counter();
c2.increment();
console.log(Counter.count);

A 0

B 2

C 1

D undefined

💡 Hint

Remember static members belong to the class, not instances.

Static member count is shared across all instances. Each increment() call increases the same count. After two increments, Counter.count is 2.

🧠 Conceptual

intermediate

Type of static member

Which of the following correctly declares a static member with a type in TypeScript?

A static name: string = "App";

B name: static string = "App";

C static name = string "App";

D static string name = "App";

💡 Hint

Static members use the static keyword before the name, then the type annotation.

Option A uses correct TypeScript syntax: static name: string = "App";. Other options have invalid order or keywords.

🔧 Debug

advanced

Why does this code cause an error?

Consider this TypeScript code snippet:

class Example {
  static value: number;
  constructor() {
    this.value = 5;
  }
}

Why does this code cause an error?

A Constructor cannot be declared in a class with static members

B Static members must be initialized outside the class

C Static members cannot be numbers

D "this" cannot be used to access static members inside constructor

💡 Hint

Think about how static members belong to the class, not instances.

Static members belong to the class itself, so inside instance methods or constructor, this refers to the instance, not the class. To access static members, use the class name.

📝 Syntax

advanced

Identify the syntax error in static member declaration

Which option contains a syntax error when declaring a static member in TypeScript?

A static count: number = 0;

B static readonly count: number = 0;

C static count number = 0;

D static count = 0;

💡 Hint

Check the order of type annotation and variable name.

Option C is invalid because it misses the colon between the variable name and type. Correct syntax is count: number.

🚀 Application

expert

Count instances using static member

What will be the value of InstanceCounter.getCount() after creating three instances?

class InstanceCounter {
  private static count: number = 0;
  constructor() {
    InstanceCounter.count++;
  }
  static getCount(): number {
    return InstanceCounter.count;
  }
}

const a = new InstanceCounter();
const b = new InstanceCounter();
const c = new InstanceCounter();
console.log(InstanceCounter.getCount());

A 3

B 0

C 1

D undefined

💡 Hint

Static members keep track of data shared by all instances.

Each time the constructor runs, it increments the static count. After three instances, count is 3.

Practice

1. Which statement about static members in TypeScript is true?

easy

A. Static members are private by default.

B. Static members can only be accessed through instances of the class.

C. Static members belong to the class itself, not to instances.

D. Static members must be declared inside the constructor.

Solution

1. Step 1: Understand what static means

   Static members belong to the class itself, not to any object created from the class.

2. Step 2: Check each option

   Static members belong to the class itself, not to instances. correctly states this. Static members can only be accessed through instances of the class. is wrong because static members are accessed via the class, not instances. Static members are private by default. is false; static members can have any access modifier. Static members must be declared inside the constructor. is incorrect; static members are declared outside the constructor.

3. Final Answer:

   Static members belong to the class itself, not to instances. -> Option C

4. Quick Check:

   Static = class-level member [OK]

Hint: Remember: static means class-level, not instance-level [OK]

Common Mistakes:

• Thinking static members belong to instances
• Trying to access static members via objects
• Assuming static members are private by default

2. Which of the following is the correct syntax to declare a static property with type number in a TypeScript class?

easy

A. static count: number = 0;

B. count static: number = 0;

C. static number count = 0;

D. number static count = 0;

Solution

1. Step 1: Recall static property syntax

   In TypeScript, static properties are declared with the keyword static before the property name and type.

2. Step 2: Check each option

   static count: number = 0; uses correct order: static count: number = 0;. Options B, C, and D have incorrect order of keywords or types.

3. Final Answer:

   static count: number = 0; -> Option A

4. Quick Check:

   static + name + : + type + = value [OK]

Hint: Static keyword always comes before property name [OK]

Common Mistakes:

• Placing type before static keyword
• Swapping order of static and property name
• Missing colon between property name and type

3. What will be the output of the following TypeScript code?

class Counter {
  static count: number = 0;
  increment() {
    Counter.count++;
  }
}
const c1 = new Counter();
const c2 = new Counter();
c1.increment();
c2.increment();
console.log(Counter.count);

medium

A. 0

B. 1

C. undefined

D. 2

Solution

1. Step 1: Understand static property usage

   The static property count belongs to the class Counter, shared by all instances.

2. Step 2: Trace the increments

   Both c1.increment() and c2.increment() increase Counter.count by 1 each, so total increments = 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Static count increments twice = 2 [OK]

Hint: Static properties are shared; increments add up [OK]

Common Mistakes:

• Thinking static count resets per instance
• Expecting output 1 instead of 2
• Confusing instance and static properties

4. Identify the error in this TypeScript class using static members:

class Example {
  static value: string;
  constructor() {
    this.value = "Hello";
  }
}

medium

A. Cannot assign to static property using this inside constructor.

B. Static properties must be initialized outside the class.

C. Static properties cannot have type annotations.

D. Constructor cannot be used with static properties.

Solution

1. Step 1: Understand static property access

   Static properties belong to the class, so they must be accessed via the class name, not this.

2. Step 2: Identify the error in constructor

   Using this.value tries to access an instance property, but value is static. Correct access is Example.value.

3. Final Answer:

   Cannot assign to static property using this inside constructor. -> Option A

4. Quick Check:

   Static accessed via class, not this [OK]

Hint: Use ClassName, not this, to access static members [OK]

Common Mistakes:

• Using this to access static properties
• Thinking static properties must be initialized outside class
• Believing constructor can't coexist with static members

5. Given this TypeScript class, how can you add a static method reset that sets count back to zero?

class Tracker {
  static count: number = 0;
  static increment() {
    this.count++;
  }
}

hard

A. Add reset() { Tracker.count = 0; } inside the class.

B. Add static reset() { this.count = 0; } inside the class.

C. Add static reset() { count = 0; } outside the class.

D. Add reset() { this.count = 0; } inside the class.

Solution

1. Step 1: Define a static method inside the class

   Static methods must be declared with the static keyword inside the class.

2. Step 2: Reset static property using this inside static method

   Inside a static method, this refers to the class, so this.count = 0; correctly resets the static property.

3. Final Answer:

   Add static reset() { this.count = 0; } inside the class. -> Option B

4. Quick Check:

   Static method + this resets static property [OK]

Hint: Static methods use static keyword and access static props via this [OK]

Common Mistakes:

• Omitting static keyword on method
• Using instance method to modify static property
• Trying to reset static property outside class without static