NonNullable type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the NonNullable<T> type do in TypeScript?

It creates a new type by removing null and undefined from the type T. This means the resulting type cannot be null or undefined.

beginner

How would you use NonNullable to exclude null and undefined from a type string | null | undefined?

You write NonNullable<string | null | undefined>, which results in just string.

beginner

True or False: NonNullable<number | null | undefined> is the same as number.

True. NonNullable removes null and undefined, leaving only number.

intermediate

Why is NonNullable useful in TypeScript?

It helps prevent errors by ensuring variables do not hold null or undefined values, which can cause runtime problems.

beginner

What happens if you apply NonNullable to a type that does not include null or undefined?

The type stays the same because there is nothing to remove.

What does NonNullable<string | null | undefined> evaluate to?

A string

B string | null

C null | undefined

D string | undefined

NonNullable removes null and undefined, leaving only string.

Which values are removed by NonNullable<T>?

A null and false

B false and 0

C undefined and 0

D null and undefined

NonNullable specifically removes null and undefined from the type.

If a variable is typed as NonNullable<number | null>, what values can it hold?

A Numbers and null

B Only numbers

C Only null

D Numbers and undefined

The NonNullable type removes null, so only numbers remain.

What is the main benefit of using NonNullable in your code?

A To avoid runtime errors from null or undefined values

B To allow more null values

C To convert types to strings

D To add undefined to types

NonNullable helps catch errors early by disallowing null and undefined.

What happens if you apply NonNullable to a type like boolean?

A It becomes undefined

B It becomes never

C The type stays as boolean

D It becomes null

Since boolean does not include null or undefined, it remains unchanged.

Explain what the NonNullable type does and give an example.

Think about how it cleans a type from empty values.

Why is it helpful to use NonNullable in TypeScript code?

Consider what problems null or undefined can cause.

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