Mapped type with conditional types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a mapped type that makes all properties optional.

Typescript

type PartialType<T> = { [P in keyof T]?: T[1] };

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

A[P]

Attempts:

3 left

💡 Hint

Common Mistakes

Using T.P instead of T[P]

Forgetting the optional modifier '?'

In a mapped type, [P in keyof T] iterates over keys of T. The syntax T[P] accesses the type of property P in T.

2fill in blank

medium

Complete the code to create a mapped type that makes properties readonly only if they are strings.

Typescript

type ReadonlyStrings<T> = { [P in keyof T]: T[P] extends string ? [1] : T[P] };

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AT[P]

Breadonly T[P]

Creadonly P

Dreadonly string

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readonly P' instead of 'readonly T[P]'

Forgetting to use 'readonly' keyword

The conditional type checks if T[P] extends string, then makes that property readonly by prefixing with 'readonly'.

3fill in blank

hard

Fix the error in the mapped type that should exclude properties of type number.

Typescript

type ExcludeNumbers<T> = { [P in keyof T as T[P] extends number ? never : [1]]: T[P] };

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Astring

Bnever

Ckeyof T

DP

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'never' instead of 'P' in the else part

Using 'keyof T' which is invalid here

The 'as' clause remaps keys. To keep keys that are not numbers, use 'P' for those keys.

4fill in blank

hard

Fill both blanks to create a mapped type that makes properties nullable only if they are objects.

Typescript

type NullableObjects<T> = { [P in keyof T]: T[P] extends object ? [1] : [2] };

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AT[P] | null

BT[P]

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Making all properties nullable regardless of type

Using 'null' alone instead of 'T[P] | null'

If T[P] is an object, we add '| null' to make it nullable; otherwise, keep the original type.

5fill in blank

hard

Fill both blanks to create a mapped type that picks only properties whose types extend string or number.

Typescript

type PickStringOrNumber<T> = { [P in keyof T as T[P] extends [1] ? P : T[P] extends [2] ? P : never]: T[P] };

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Astring

Bnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'boolean' instead of 'number'

Adding extra modifiers to property types

The mapped type uses conditional keys to pick properties of type string or number. The third blank is empty because no modification is needed to the property type.