Generic interface declaration in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generic interface usage

What is the output of this TypeScript code when compiled and run with Node.js (ignoring type checks)?

Typescript

interface Box<T> {
  content: T;
}

const box: Box<number> = { content: 123 };
console.log(box.content);

A 123

B undefined

C Error: Type 'string' is not assignable to type 'number'.

D null

💡 Hint

Look at the value assigned to content and what is logged.

The interface Box defines a generic type with a content property of type T. Here T is number, so content is 123. Logging box.content prints 123.

❓ Predict Output

intermediate

Output of generic interface with union type

What will be the output of this code snippet?

Typescript

interface Result<T> {
  success: boolean;
  data: T;
}

const res: Result<string | null> = { success: true, data: null };
console.log(res.data);

A Error: Type 'null' is not assignable to type 'string'.

B undefined

C "null"

D null

💡 Hint

Check the assigned value and the union type.

The data property can be string or null. Here it is null, so logging prints null.

🔧 Debug

advanced

Identify the error in generic interface usage

What error will this TypeScript code produce?

Typescript

interface Pair<T, U> {
  first: T;
  second: U;
}

const pair: Pair<number> = { first: 1, second: 2 };

A Error: Property 'second' is missing in type '{ first: number; }'.

B Error: Generic type 'Pair<T, U>' requires 2 type arguments.

C Error: Type 'number' is not assignable to type 'U'.

D No error, compiles fine.

💡 Hint

Check how many generic parameters Pair expects.

Pair expects two generic types T and U, but only one is provided. This causes a compile-time error.

❓ Predict Output

advanced

Output of nested generic interfaces

What is the output of this code?

Typescript

interface Container<T> {
  value: T;
}

interface Wrapper<U> {
  container: Container<U>;
}

const wrapped: Wrapper<string> = { container: { value: "hello" } };
console.log(wrapped.container.value);

A "hello"

B undefined

C Error: Type 'number' is not assignable to type 'string'.

D null

💡 Hint

Look at the nested generic types and the assigned values.

Wrapper has a container property of type Container. Here U is string, so container.value is "hello". Logging it prints "hello".

🧠 Conceptual

expert

Number of properties in a generic interface instance

Given this generic interface and instance, how many properties does the object 'item' have at runtime?

Typescript

interface Item<T> {
  id: number;
  data: T;
  timestamp?: Date;
}

const item: Item<string> = { id: 1, data: "abc" };

A 1

B 3

C 2

D 0

💡 Hint

Optional properties may not exist on the object.

The object has 'id' and 'data' properties defined. 'timestamp' is optional and not present, so total properties are 2.