NonNullable type in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove null and undefined from the type using NonNullable.

Typescript

type CleanType = NonNullable<[1]>;

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

Anull | undefined

Bnumber | null

Cstring | null | undefined

Dboolean | undefined

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a type that does not include both null and undefined.

Confusing NonNullable with Partial or Required.

The NonNullable type removes null and undefined from the given type. Here, it removes them from string | null | undefined, leaving just string.

2fill in blank

medium

Complete the code to define a variable that cannot be null or undefined using NonNullable.

Typescript

let value: NonNullable<[1]> = "hello";

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

Astring | undefined

Bstring | null | undefined

Cstring | null

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type that only has null or only undefined, not both.

Trying to assign null or undefined to the variable.

The type string | null | undefined includes both null and undefined. Using NonNullable removes these, so value can only be a string.

3fill in blank

hard

Fix the error in the code by using NonNullable correctly.

Typescript

function greet(name: [1]) {
  console.log(`Hello, ${name}!`);
}
greet(null);

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

ANonNullable<string | null>

Bstring | undefined

Cstring | null

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using just string | null which allows null.

Using string without considering the original type.

The function parameter name should not accept null. Using NonNullable removes null, so the function only accepts strings.

4fill in blank

hard

Fill both blanks to create a type that excludes null and undefined from a union type.

Typescript

type Cleaned = NonNullable<[1] | [2]>;

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

Astring

Bnull

Cundefined

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined instead of null in the union.

Putting both blanks as normal types.

The union string | null includes null. Using NonNullable removes null, leaving just string.

5fill in blank

hard

Fill all three blanks to define a variable type that excludes null and undefined from a union.

Typescript

let data: NonNullable<[1] | [2] | [3]>;

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

Astring

Bnull

Cundefined

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving out either null or undefined in the union.

Trying to assign null or undefined to the variable.

The union string | null | undefined includes both null and undefined. Using NonNullable removes these, so data can only be a string.

Practice

1.

What does the NonNullable type do in TypeScript?

easy

A. Converts a type to a string

B. Adds null and undefined to a type

C. Removes null and undefined from a type

D. Makes a type optional

Solution

1. Step 1: Understand the purpose of NonNullable

   The NonNullable type removes null and undefined from a given type, ensuring the value is always defined.

2. Step 2: Compare options with definition

   Only Removes null and undefined from a type correctly describes this behavior; others describe different or incorrect behaviors.

3. Final Answer:

   Removes null and undefined from a type -> Option C

4. Quick Check:

   NonNullable removes null and undefined [OK]

Hint: Remember: NonNullable cuts out null and undefined [OK]

Common Mistakes:

• Thinking it adds null or undefined
• Confusing it with making types optional
• Assuming it converts types to strings

2.

Which of the following is the correct syntax to use NonNullable on a type T?

easy

A. NonNullable(T)

B. NonNullable{T}

C. NonNullable[T]

D. NonNullable<T>

Solution

1. Step 1: Recall TypeScript generic syntax

   TypeScript uses angle brackets <> to apply generic types, like NonNullable<T>.

2. Step 2: Match options with correct syntax

   NonNullable<T> uses the correct generic syntax. Options A, C, and D use incorrect brackets or parentheses.

3. Final Answer:

   NonNullable<T> -> Option D

4. Quick Check:

   Generic types use <> brackets [OK]

Hint: Use angle brackets for generics: NonNullable<T> [OK]

Common Mistakes:

• Using parentheses instead of angle brackets
• Using square brackets or curly braces
• Confusing function call syntax with type syntax

3.

What is the type of value after this code runs?

type MaybeString = string | null | undefined;
const value: NonNullable<MaybeString> = "hello";

medium

A. string

B. null | undefined

C. string | null | undefined

D. never

Solution

1. Step 1: Understand the original type

   MaybeString can be string, null, or undefined.

2. Step 2: Apply NonNullable to remove null and undefined

   Using NonNullable<MaybeString> removes null and undefined, leaving only string.

3. Final Answer:

   string -> Option A

4. Quick Check:

   NonNullable removes null and undefined, leaving string [OK]

Hint: NonNullable removes null and undefined from union types [OK]

Common Mistakes:

• Thinking the type still includes null or undefined
• Confusing NonNullable with Partial or Required
• Assuming the type becomes never

4.

Identify the error in this code snippet:

type User = { name: string | null };
type CleanUser = NonNullable<User>;

medium

A. There is no error; code is correct

B. The syntax for NonNullable is incorrect

C. NonNullable removes null from properties inside objects automatically

D. NonNullable cannot be applied to object types directly

Solution

1. Step 1: Understand what NonNullable does

   NonNullable removes null and undefined from a type itself, not from nested properties.

2. Step 2: Analyze the code

   The code is syntactically correct. NonNullable<User> results in the same type as User since User is not a union including null or undefined at the top level.

3. Final Answer:

   There is no error; code is correct -> Option A

4. Quick Check:

   Code compiles without errors [OK]

Hint: NonNullable can be applied to object types but does not clean nested properties [OK]

Common Mistakes:

• Believing NonNullable cleans nested properties automatically
• Thinking the syntax for NonNullable is incorrect
• Thinking NonNullable cannot be applied to object types

5.

Given this type, how can you create a new type NonNullUser where the email property cannot be null or undefined using NonNullable?

type User = {
  id: number;
  email: string | null | undefined;
};

hard

A. type NonNullUser = NonNullable<User>

B. type NonNullUser = { id: number; email: NonNullable<string | null | undefined> }

C. type NonNullUser = { id: number; email: string }

D. type NonNullUser = { id: number; email: string | null }

Solution

1. Step 1: Understand the goal

   We want to keep id as is, but make email exclude null and undefined.

2. Step 2: Apply NonNullable to the email property only

   Using NonNullable<string | null | undefined> on email removes null and undefined, leaving just string.

3. Step 3: Check other options

   type NonNullUser = NonNullable<User> removes null/undefined from the whole User type, which does not affect nested properties. type NonNullUser = { id: number; email: string } changes the type but does not use NonNullable. type NonNullUser = { id: number; email: string | null } still allows null.

4. Final Answer:

   type NonNullUser = { id: number; email: NonNullable<string | null | undefined> } -> Option B

5. Quick Check:

   Apply NonNullable only to the property needing it [OK]

Hint: Use NonNullable on specific properties, not whole object [OK]

Common Mistakes:

• Applying NonNullable to the whole object type
• Not removing null and undefined from email
• Changing type without NonNullable