Why advanced generics matter in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic function that returns the input value.

Typescript

function identity<T>([1]: T): T {
  return value;
}

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

Aitem

Bvalue

Cinput

Darg

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name different from the one returned causes errors.

The generic function uses value as the parameter name to return it directly.

2fill in blank

medium

Complete the code to define a generic interface with a property of type T.

Typescript

interface Box<[1]> {
  contents: T;
}

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

AU

BK

CV

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic parameter name that does not match the property type causes errors.

The generic type parameter is conventionally named T and used inside the interface.

3fill in blank

hard

Fix the error in the generic function to constrain T to types with a length property.

Typescript

function logLength<T [1] { length: number }>(arg: T): T {
  console.log(arg.length);
  return arg;
}

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

Ainherits

Bimplements

Cextends

Dconforms

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect keywords like 'implements' or 'inherits' causes syntax errors.

The extends keyword constrains the generic type to types that have a length property.

4fill in blank

hard

Fill both blanks to create a generic function that returns the first element of an array of type T.

Typescript

function firstElement<[1]>(arr: [2][]): [1] {
  return arr[0];
}

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

AT

BU

DV

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic letters causes type mismatch errors.

The generic type parameter T is used both to declare the generic and the array element type.

5fill in blank

hard

Fill all three blanks to create a generic function that maps an array of type T to an array of type U using a callback.

Typescript

function mapArray<[1], [2]>(arr: [1][], callback: (item: [1]) => [2]): [2][] {
  return arr.map(callback);
}

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

AT

BU

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up generic types causes type errors in the callback or return value.

The generic types T and U represent input and output types respectively, used consistently in parameters and return type.

Practice

1. What is the main benefit of using advanced generics in TypeScript?

easy

A. They automatically fix bugs in your code.

B. They make the code run faster at runtime.

C. They allow writing flexible and reusable code with type safety.

D. They replace the need for functions.

Solution

1. Step 1: Understand what generics do

   Generics let you write code that works with many types without losing type safety.

2. Step 2: Identify the main benefit

   Advanced generics improve flexibility and reusability while preventing type errors.

3. Final Answer:

   They allow writing flexible and reusable code with type safety. -> Option C

4. Quick Check:

   Advanced generics = flexible reusable type-safe code [OK]

Hint: Think: generics = flexible + safe code [OK]

Common Mistakes:

• Confusing generics with runtime performance improvements
• Believing generics fix bugs automatically
• Thinking generics replace functions

2. Which of the following is the correct syntax to declare a generic function in TypeScript?

easy

A. function example(arg: T): T { return arg; }

B. function example<T>(arg: T): void { return arg; }

C. function example<T>(arg): T { return arg; }

D. function example<T>(arg: T): T { return arg; }

Solution

1. Step 1: Check generic syntax

   Generic functions declare a type parameter inside angle brackets after the function name.

2. Step 2: Verify parameter and return types

   The argument and return type should use the generic type parameter T.

3. Final Answer:

   function example<T>(arg: T): T { return arg; } -> Option D

4. Quick Check:

   Generic function syntax = function name<T>(arg: T): T [OK]

Hint: Generic syntax: function name<T>(arg: T): T [OK]

Common Mistakes:

• Omitting the generic parameter declaration <T>
• Not typing the argument with T
• Returning a value in a void function

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

function wrapInArray<T>(value: T): T[] {
  return [value];
}

const result = wrapInArray(42);
console.log(result);

medium

A. [42]

B. 42

C. undefined

D. Error: Type mismatch

Solution

1. Step 1: Understand the function behavior

   The function takes a value of any type T and returns an array containing that value.

2. Step 2: Apply the function to the input

   Calling wrapInArray(42) returns [42], an array with the number 42 inside.

3. Final Answer:

   [42] -> Option A

4. Quick Check:

   wrapInArray(42) = [42] [OK]

Hint: Generic function returns array with input inside [OK]

Common Mistakes:

• Expecting just the value instead of an array
• Thinking it returns undefined
• Confusing type errors with runtime output

4. Identify the error in this TypeScript generic function:

function identity<T>(arg: T): T {
  return arg.length;
}

medium

A. The function returns a number but declares return type T.

B. The generic type T is not declared correctly.

C. The argument arg should not be generic.

D. There is no error in this function.

Solution

1. Step 1: Analyze the return statement

   The function returns arg.length, which is a number.

2. Step 2: Check return type consistency

   The function declares it returns type T, but number may not be T, causing a type mismatch.

3. Final Answer:

   The function returns a number but declares return type T. -> Option A

4. Quick Check:

   Return type mismatch = error [OK]

Hint: Return type must match declared generic type [OK]

Common Mistakes:

• Assuming arg always has length property
• Ignoring return type mismatch
• Thinking generic declaration is wrong

5. Given this generic interface and function, what is the correct way to use advanced generics to ensure type safety?

interface Container<T> {
  value: T;
}

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

const numberContainer = { value: 123 };
const stringContainer = { value: "hello" };

// Which call is correct?

hard

A. getValue(stringContainer);

B. getValue<number>(numberContainer);

C. getValue<number>(stringContainer);

D. getValue<string>(numberContainer);

Solution

1. Step 1: Match generic type with container value

   numberContainer has value of type number, so T should be number.

2. Step 2: Check each option for type safety

   getValue<number>(numberContainer); uses getValue<number>(numberContainer), which matches types correctly.

3. Final Answer:

   getValue<number>(numberContainer); -> Option B

4. Quick Check:

   Generic type matches container value type [OK]

Hint: Generic type must match container's value type [OK]

Common Mistakes:

• Passing wrong generic type for container value
• Omitting generic type when explicit needed
• Mixing container types incorrectly