Mapped type for deep transformations in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mapped type in TypeScript?

A mapped type creates a new type by transforming each property of an existing type. It uses syntax like { [K in keyof T]: ... } to loop over keys.

intermediate

How can you apply a transformation deeply to nested objects using mapped types?

You can use recursive mapped types that check if a property is an object and then apply the mapped type again to that property, enabling deep transformations.

advanced

Explain the purpose of the infer keyword in deep mapped types.

infer helps extract types inside conditional types. It can be used to capture inner types during recursion for deep transformations.

intermediate

What does this TypeScript type do?

type DeepReadonly<T> = { readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K] };

It makes all properties of T deeply readonly, meaning nested objects' properties are also readonly recursively.

intermediate

Why do we need to check if a property is an object before applying deep mapped types?

Because primitive types like string or number should not be transformed recursively. Checking prevents errors and infinite recursion.

What does keyof T represent in a mapped type?

A A primitive type

B All values of type T

C A function in T

D All keys of type T

keyof T is a union of all property names (keys) of type T.

Which TypeScript feature allows a mapped type to apply itself recursively?

A Enums

B Conditional types

C Interfaces

D Type assertions

Conditional types let you check if a property is an object and then apply the mapped type recursively.

What is the main risk if you don't check for object types in deep mapped types?

A Infinite recursion

B Syntax error

C TypeScript crashes

D No effect

Without checking, recursion can continue infinitely causing errors.

What does the readonly modifier do in a mapped type?

A Makes properties optional

B Deletes properties

C Prevents properties from being changed

D Changes property types

readonly makes properties immutable after initialization.

Which syntax correctly defines a mapped type?

A { [K in keyof T]: T[K] }

B function map(T) { return T; }

C interface Mapped<T> {}

D type Mapped = T[]

Mapped types use { [K in keyof T]: ... } syntax to transform properties.

Describe how to create a deep transformation mapped type in TypeScript.

Think about looping over keys and applying the type again if the property is an object.

Explain why deep mapped types are useful in real-world TypeScript projects.

Consider how nested data structures benefit from consistent type changes.

Practice

1.

What is the main purpose of a mapped type in TypeScript?

easy

A. To create a new type by transforming each property of an existing type

B. To execute functions asynchronously

C. To define runtime values for variables

D. To handle errors in promises

Solution

1. Step 1: Understand mapped types

   Mapped types create new types by applying transformations to each property of an existing type.

2. Step 2: Compare options

   Only To create a new type by transforming each property of an existing type describes this behavior correctly; others relate to runtime or async concepts.

3. Final Answer:

   To create a new type by transforming each property of an existing type -> Option A

4. Quick Check:

   Mapped type = transform properties [OK]

Hint: Mapped types change keys' types one by one [OK]

Common Mistakes:

• Confusing mapped types with runtime functions
• Thinking mapped types handle async code
• Mixing mapped types with error handling

2.

Which of the following is the correct syntax to define a simple mapped type that makes all properties of T optional?

type PartialType = ?

easy

A. type PartialType<T> = { [P in keyof T]: T[P] }

B. type PartialType<T> = { [P in keyof T]: Optional }

C. type PartialType<T> = { [P in keyof T]?: T[P] }

D. type PartialType<T> = { [P: string]: T }

Solution

1. Step 1: Recall optional property syntax

   Adding a question mark after the property name makes it optional in mapped types.

2. Step 2: Analyze options

   type PartialType<T> = { [P in keyof T]?: T[P] } uses ?: correctly to make properties optional. type PartialType<T> = { [P in keyof T]: T[P] } keeps properties required. type PartialType<T> = { [P: string]: T } uses string keys incorrectly. type PartialType<T> = { [P in keyof T]: Optional } uses a non-existent Optional type.

3. Final Answer:

   type PartialType<T> = { [P in keyof T]?: T[P] } -> Option C

4. Quick Check:

   Optional properties use ? after key [OK]

Hint: Use ? after [P in keyof T] to make properties optional [OK]

Common Mistakes:

• Forgetting the ? to mark properties optional
• Using string index instead of keyof T
• Assuming Optional<T> is built-in

3.

Given the following code, what is the type of DeepReadonly<User>?

type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object ? DeepReadonly<T[P]> : T[P]
};

interface User {
  name: string;
  address: {
    city: string;
    zip: number;
  };
}

medium

A. { name: string; readonly address: { readonly city: string; readonly zip: number; } }

B. { name: string; address: { city: string; zip: number; } }

C. { readonly name: string; address: { city: string; zip: number; } }

D. { readonly name: string; readonly address: { readonly city: string; readonly zip: number; } }

Solution

1. Step 1: Understand DeepReadonly mapped type

   It recursively applies readonly to all properties, including nested objects.

2. Step 2: Apply to User interface

   Both name and address become readonly, and inside address, city and zip are also readonly.

3. Final Answer:

   { readonly name: string; readonly address: { readonly city: string; readonly zip: number; } } -> Option D

4. Quick Check:

   DeepReadonly = recursive readonly [OK]

Hint: DeepReadonly adds readonly recursively to all nested properties [OK]

Common Mistakes:

• Forgetting readonly on nested properties
• Assuming only top-level properties are readonly
• Confusing shallow and deep readonly

4.

Identify the error in this deep transformation mapped type:

type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends object ? DeepPartial<T[P]> : T[P];
}

medium

A. Incorrect use of semicolon instead of comma in mapped type

B. No error; the type is correct

C. The conditional type should check for primitives, not objects

D. Missing semicolon after the mapped property

Solution

1. Step 1: Check mapped type syntax

   TypeScript object types, including mapped types, accept both commas and semicolons as separators, including trailing ones.

2. Step 2: Analyze the code

   The syntax is valid, and the logic correctly makes properties optional while recursing DeepPartial on nested objects and preserving primitives.

3. Final Answer:

   No error; the type is correct -> Option B

4. Quick Check:

   Mapped types accept , or ; [OK]

Hint: Mapped types support , or ; separators [OK]

Common Mistakes:

• Thinking semicolons are invalid in mapped types
• Confusing separator rules from object literals
• Overlooking recursive conditional logic

5.

How would you write a mapped type DeepNonNullable<T> that recursively removes null and undefined from all properties of T, including nested objects?

hard

A. type DeepNonNullable<T> = { [P in keyof T]: T[P] extends object ? DeepNonNullable<NonNullable<T[P]>> : NonNullable<T[P]> }

B. type DeepNonNullable<T> = { [P in keyof T]: NonNullable<T[P]> }

C. type DeepNonNullable<T> = { [P in keyof T]: T[P] extends object ? NonNullable<DeepNonNullable<T[P]>> : T[P] }

D. type DeepNonNullable<T> = { [P in keyof T]: T[P] extends object ? DeepNonNullable<T[P]> : T[P] }

Solution

1. Step 1: Understand NonNullable and recursion

   We must remove null and undefined from each property, then recurse if the property is an object.

2. Step 2: Analyze options

   type DeepNonNullable<T> = { [P in keyof T]: T[P] extends object ? DeepNonNullable<NonNullable<T[P]>> : NonNullable<T[P]> } first applies NonNullable to the property, then recursively applies DeepNonNullable to the cleaned type. This ensures null/undefined are removed at every level.

3. Final Answer:

   type DeepNonNullable<T> = { [P in keyof T]: T[P] extends object ? DeepNonNullable<NonNullable<T[P]>> : NonNullable<T[P]> } -> Option A

4. Quick Check:

   Remove null then recurse for objects [OK]

Hint: Apply NonNullable before recursion for deep cleaning [OK]

Common Mistakes:

• Not applying NonNullable before recursion
• Recursing without removing null/undefined
• Confusing order of conditional checks