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.