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.

Practice

1. What does the <T> represent in a TypeScript generic interface declaration?

easy

A. A special type that only accepts strings

B. A variable name for a function

C. A keyword to declare a constant

D. A placeholder for a type that will be specified later

Solution

1. Step 1: Understand generic syntax

   The <T> syntax in TypeScript is used to declare a generic type parameter, which acts as a placeholder for a type.

2. Step 2: Identify the role of T

   This placeholder allows the interface to be flexible and reusable with different types specified later.

3. Final Answer:

   A placeholder for a type that will be specified later -> Option D

4. Quick Check:

   Generic <T> = Type placeholder [OK]

Hint: Remember <T> means 'type to be given later' [OK]

Common Mistakes:

• Thinking T is a variable or constant
• Confusing <T> with a function parameter
• Assuming T is always a string type

2. Which of the following is the correct syntax to declare a generic interface named Box with a type parameter T?

easy

A. interface Box<T> = { content: T; }

B. interface Box<T> { content: T; }

C. interface Box<T> content: T;

D. interface Box { content: T; }

Solution

1. Step 1: Check interface declaration syntax

   In TypeScript, interfaces are declared with the keyword interface followed by the name and optional generic parameters in angle brackets.

2. Step 2: Verify correct use of braces and type parameter

   The correct syntax includes curly braces to define the interface body and uses <T> to declare the generic type parameter.

3. Final Answer:

   interface Box<T> { content: T; } -> Option B

4. Quick Check:

   Generic interface syntax = interface Box<T> { content: T; } [OK]

Hint: Generic interfaces need <T> and braces {} [OK]

Common Mistakes:

• Omitting the generic parameter <T>
• Missing curly braces around interface body
• Using equals sign instead of braces

3. Given the interface declaration:

interface Container<T> { value: T; }

What is the output of this code?

const numberContainer: Container<number> = { value: 42 };
console.log(numberContainer.value);

medium

A. Error: Type mismatch

B. undefined

C. 42

D. null

Solution

1. Step 1: Understand the generic interface usage

   The interface Container<T> expects a type parameter. Here, number is specified, so value must be a number.

2. Step 2: Analyze the assigned object and console output

   The object { value: 42 } matches the interface with value as a number. Logging numberContainer.value prints 42.

3. Final Answer:

   42 -> Option C

4. Quick Check:

   Generic with number type outputs 42 [OK]

Hint: Generic type sets property type; output matches assigned value [OK]

Common Mistakes:

• Expecting a runtime error due to generics
• Confusing generic type with runtime type enforcement
• Assuming output is undefined or null

4. Identify the error in this generic interface usage:

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

const pair: Pair = { first: 'hello', second: 'world' };

medium

A. Missing generic type argument when using Pair

B. Properties first and second should be numbers

C. Interface Pair cannot have two properties of the same type

D. Object literal syntax is incorrect

Solution

1. Step 1: Check interface usage with generics

   The interface Pair<T> requires a type argument when used. Here, Pair is used without specifying <T>.

2. Step 2: Identify the missing generic argument error

   TypeScript will report an error because the generic type parameter is missing when declaring pair.

3. Final Answer:

   Missing generic type argument when using Pair -> Option A

4. Quick Check:

   Generic interface usage needs type argument [OK]

Hint: Always specify type argument when using generic interfaces [OK]

Common Mistakes:

• Forgetting to add <Type> when using generic interfaces
• Assuming default type if omitted
• Confusing property types with generic parameter

5. You want to create a generic interface Result<T> that holds a data property of type T and a success boolean. Which of the following implementations correctly uses this interface to type a variable holding a string result?

hard

A. const res: Result<string> = { data: 'Done', success: true };

B. const res: Result<string> = { data: 123, success: true };

C. const res: Result = { data: 'Done', success: true };

D. const res: Result<number> = { data: 'Done', success: true };

Solution

1. Step 1: Understand the generic interface requirements

   The interface Result<T> expects a data property of type T and a success boolean.

2. Step 2: Check each option for correct type usage

   const res: Result<string> = { data: 123, success: true }; assigns a number to data but expects a string type, so it's incorrect.
   const res: Result<string> = { data: 'Done', success: true }; correctly assigns a string to data and includes success.
   const res: Result = { data: 'Done', success: true }; omits the generic type argument, which is required.
   const res: Result<number> = { data: 'Done', success: true }; uses number type but assigns a string, causing a type mismatch.

3. Final Answer:

   const res: Result<string> = { data: 'Done', success: true }; -> Option A

4. Quick Check:

   Generic type matches property type = const res: Result<string> = { data: 'Done', success: true }; [OK]

Hint: Match generic type with property value type exactly [OK]

Common Mistakes:

• Assigning wrong type to generic property
• Omitting generic type argument
• Mixing up generic type parameters