Why generic interfaces matter in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic interface named Container.

Typescript

interface Container<[1]> {
  value: T;
}

Drag options to blanks, or click blank then click option'

AV

BU

CT

DX

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase letters for generic type parameters.

Forgetting to add the generic parameter in angle brackets.

The generic type parameter T is used to define the type inside the interface.

2fill in blank

medium

Complete the code to create a variable of type Container holding a number.

Typescript

const numberContainer: Container<[1]> = { value: 42 };

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using string when the value is a number.

Using any which is less specific.

We specify number as the generic type to hold a number value.

3fill in blank

hard

Fix the error in the function signature to accept a generic Container.

Typescript

function getValue<[1]>(container: Container<T>): T {
  return container.value;
}

Drag options to blanks, or click blank then click option'

AT

BV

CU

DW

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different generic letter in the function than in the interface.

Omitting the generic parameter in the function.

The generic parameter T must match the one used in the parameter type Container<T>.

4fill in blank

hard

Fill both blanks to create a generic interface Pair with two types.

Typescript

interface Pair<[1], [2]> {
  first: [1];
  second: [2];
}

Drag options to blanks, or click blank then click option'

AA

BB

CT

DU

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same letter for both generic parameters.

Using letters that do not match the property types.

The generic parameters A and B represent the types of the two values in the pair.

5fill in blank

hard

Fill all three blanks to create a function that returns the first element of a generic Pair.

Typescript

function getFirst<[1], [2]>(pair: Pair<[3], B>): A {
  return pair.first;
}

Drag options to blanks, or click blank then click option'

AA

BB

CT

DU

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up generic parameter letters.

Not matching the generic parameters in the function signature and parameter.

The function uses generic parameters A and B matching the Pair interface. The parameter pair is typed as Pair<A, B> and the return type is A.

Practice

1. What is the main benefit of using generic interfaces in TypeScript?

easy

A. They automatically generate user interfaces.

B. They make the code run faster at runtime.

C. They allow the same interface to work with different data types.

D. They replace the need for classes.

Solution

1. Step 1: Understand what generic interfaces do

   Generic interfaces let you define a structure that can work with any type, not just one fixed type.

2. Step 2: Identify the benefit

   This flexibility means you can reuse the same interface for different data types, making your code more flexible and reusable.

3. Final Answer:

   They allow the same interface to work with different data types. -> Option C

4. Quick Check:

   Generic interfaces = flexible types [OK]

Hint: Generic interfaces = one interface, many types [OK]

Common Mistakes:

• Thinking generics improve runtime speed
• Confusing generics with UI generation
• Believing generics replace classes

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 { content: T; }

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

C. interface Box<T> { content: any; }

D. interface Box<T> content: T;

Solution

1. Step 1: Recall generic interface syntax

   Generic interfaces use angle brackets after the name to declare type parameters, like interface Name<T>.

2. Step 2: Check the property type

   The property should use the generic type parameter T to be flexible.

3. Final Answer:

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

4. Quick Check:

   Generic syntax = interface Name<T> { prop: T } [OK]

Hint: Generic interfaces use <T> after name [OK]

Common Mistakes:

• Omitting <T> after interface name
• Using any instead of generic type
• Missing curly braces or semicolons

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

interface Container<T> {
  value: T;
}

const numberContainer: Container<number> = { value: 42 };
const stringContainer: Container<string> = { value: 'hello' };

console.log(numberContainer.value);
console.log(stringContainer.value);

medium

A. 42 and hello

B. number and string

C. undefined and undefined

D. Error: Type mismatch

Solution

1. Step 1: Understand the generic interface usage

   The interface Container<T> holds a value of type T. Here, numberContainer uses number and stringContainer uses string.

2. Step 2: Check the console.log outputs

   Logging numberContainer.value prints 42, and stringContainer.value prints 'hello'.

3. Final Answer:

   42 and hello -> Option A

4. Quick Check:

   Generic interface values print correctly [OK]

Hint: Generic types keep values typed and accessible [OK]

Common Mistakes:

• Expecting type names instead of values
• Thinking generics cause runtime errors
• Confusing property access syntax

4. Identify the error in this generic interface usage:

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

const pair: Pair = { first: 1, second: 'two' };

medium

A. Syntax error in object declaration

B. Cannot mix number and string in Pair

C. Interface Pair cannot have two properties

D. Missing type argument when using Pair

Solution

1. Step 1: Check generic interface usage

   The interface Pair<T> requires a type argument when used, like Pair<number>.

2. Step 2: Identify the error in the code

   The variable pair is declared as Pair without specifying <T>, causing a TypeScript error.

3. Final Answer:

   Missing type argument when using Pair -> Option D

4. Quick Check:

   Generic interfaces need type arguments [OK]

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

Common Mistakes:

• Omitting the generic type argument
• Trying to mix types without union
• Assuming interface limits property count

5. Given this generic interface and usage, what is the best way to ensure the data property can hold either a string or an array of strings without losing type safety?

interface Response<T> {
  data: T;
}

const response1: Response<string> = { data: 'hello' };
const response2: Response = { data: ['hello', 'world'] };

// How to declare a variable that can hold either response1 or response2?

hard

A. Use a union type: Response

B. Use any type: Response<any>

C. Create two separate interfaces for string and string[]

D. Use Response<object> to cover both

Solution

1. Step 1: Understand the need for flexible but safe typing

   We want a variable that can hold either a string or an array of strings, keeping type safety.

2. Step 2: Use union types with generics

   Declaring Response<string | string[]> allows data to be either type safely.

3. Final Answer:

   Use a union type: Response<string | string[]> -> Option A

4. Quick Check:

   Union types + generics = flexible and safe [OK]

Hint: Use union types inside generics for multiple allowed types [OK]

Common Mistakes:

• Using any loses type safety
• Creating duplicate interfaces wastes code
• Using object is too broad and unsafe