Default generic types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a generic function with a default type.

Typescript

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

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

Aboolean

Bnumber

Cany

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a specific type like string or number as default when flexibility is needed.

The default generic type is any, so if no type is specified, T defaults to any.

2fill in blank

medium

Complete the code to create a generic class with a default type.

Typescript

class Box<T = [1]> {
  contents: T;
  constructor(value: T) {
    this.contents = value;
  }
}

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

Astring

Bnumber

Cboolean

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean when the example expects string.

The default generic type is string, so if no type is specified, T defaults to string.

3fill in blank

hard

Fix the error in the generic function by adding a default type.

Typescript

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

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

A extends string

B extends number

C = number

D = string

Attempts:

3 left

💡 Hint

Common Mistakes

Using extends instead of = for default types.

Adding = string sets the default generic type to string, fixing the error if no type is provided.

4fill in blank

hard

Fill both blanks to create a generic interface with a default type and a property using that type.

Typescript

interface Container<T = [1]> {
  value: [2];
}

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

Anumber

Bstring

CT

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using the property type as a concrete type instead of the generic parameter.

The default generic type is string, and the property value uses the generic type T.

5fill in blank

hard

Fill all three blanks to define a generic function with two type parameters, one with a default, and return a tuple.

Typescript

function pair<T, U = [1]>(first: T, second: U): [[2], [3]] {
  return [first, second];
}

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

Astring

BT

CU

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of generic parameters in the return type.

The second generic type U defaults to number. The return tuple uses types T and U.

Practice

1.

What is the main purpose of default generic types in TypeScript?

easy

A. To provide a fallback type when no type argument is specified

B. To enforce a specific type always

C. To disable generics in a function

D. To convert types to strings automatically

Solution

1. Step 1: Understand generic types

   Generic types allow functions or classes to work with any type specified by the user.

2. Step 2: Role of default generic types

   Default generic types provide a fallback type if the user does not specify one explicitly.

3. Final Answer:

   To provide a fallback type when no type argument is specified -> Option A

4. Quick Check:

   Default generic types = fallback type [OK]

Hint: Defaults fill in missing generic types automatically [OK]

Common Mistakes:

• Thinking default generics enforce a fixed type
• Confusing default generics with type conversion
• Believing default generics disable generics

2.

Which of the following is the correct syntax to set a default generic type T to string in a function?

function example<T = string>()

easy

A. function example<T extends string>()

B. function example<T: string>()

C. function example<T = string>()

D. function example<T? = string>()

Solution

1. Step 1: Recall TypeScript generic syntax

   Default generic types use the equals sign (=) to assign a fallback type.

2. Step 2: Analyze options

   function example<T = string>() uses <T = string> which is correct syntax for default generic types.

3. Final Answer:

   function example<T = string>() -> Option C

4. Quick Check:

   Default generic syntax uses = [OK]

Hint: Use = to assign default generic types [OK]

Common Mistakes:

• Using colon (:) instead of equals (=)
• Adding question mark (?) incorrectly
• Confusing extends with default assignment

3.

What will be the output of the following TypeScript code?

function wrap<T = number>(value: T): T[] {
  return [value];
}

const result = wrap('hello');
console.log(result);

medium

A. ["number"]

B. ["hello"]

C. ["value"]

D. Type error

Solution

1. Step 1: Understand default generic type usage

   The function wrap has a default generic type T = number, but the caller passes a string 'hello', so T becomes string.

2. Step 2: Analyze function output

   The function returns an array containing the passed value, so it returns ["hello"].

3. Final Answer:

   ["hello"] -> Option B

4. Quick Check:

   Explicit type overrides default generic [OK]

Hint: Passed type overrides default generic type [OK]

Common Mistakes:

• Assuming default generic always applies
• Expecting a type error for different type
• Confusing value with type name

4.

Identify the error in this TypeScript code using default generic types:

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

medium

A. Property 'length' may not exist on type 'T'

B. No error, code is correct

C. Return type should be number, not T

D. Default generic type cannot be 'number'

Solution

1. Step 1: Analyze the generic type and usage

   The generic type T defaults to number, but number does not have a length property.

2. Step 2: Identify the error

   Accessing arg.length causes a TypeScript error because length may not exist on type T.

3. Final Answer:

   Property 'length' may not exist on type 'T' -> Option A

4. Quick Check:

   Accessing unknown properties on generic types causes errors [OK]

Hint: Check if generic type supports accessed properties [OK]

Common Mistakes:

• Ignoring property existence on generic types
• Thinking default generic restricts properties
• Assuming no error if default is number

5.

Given this generic function with a default type, how can you call it to get an array of booleans?

function createArray<T = string>(length: number, value: T): T[] {
  return Array(length).fill(value);
}

Choose the correct call:

hard

A. createArray<boolean>(true, 3)

B. createArray(3, 'true')

C. createArray<number>(3, true)

D. createArray<boolean>(3, true)

Solution

1. Step 1: Understand default generic and call signature

   The function defaults T to string, but to get an array of booleans, we must specify T = boolean.

2. Step 2: Analyze options

   createArray<boolean>(3, true) explicitly sets T to boolean and passes true as value, which matches the type.

3. Final Answer:

   createArray<boolean>(3, true) -> Option D

4. Quick Check:

   Specify generic type to override default [OK]

Hint: Specify generic type to override default generic [OK]

Common Mistakes:

• Relying on default generic without specifying type
• Passing arguments in wrong order
• Mismatching generic type and value type