Why mapped types are needed in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Typescript

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

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A[P]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use keyof to get keys.

Using incorrect syntax for mapped types.

The syntax [P in keyof T] iterates over all keys of T to create a mapped type.

2fill in blank

medium

Complete the code to create a mapped type that makes all properties readonly.

Typescript

type ReadonlyType<T> = { readonly [P in keyof T][1] T[P] };

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A=

B:

C=>

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals = instead of colon :.

Using arrow => which is for functions.

The colon : separates the property name from its type in mapped types.

3fill in blank

hard

Fix the error in the mapped type that tries to change all properties to string.

Typescript

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

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Anumber

BT[P]

Cany

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using T[P] which keeps original types.

Using unrelated types like number.

To change all property types to string, assign string as the type.

4fill in blank

hard

Fill both blanks to create a mapped type that makes all properties nullable.

Typescript

type Nullable<T> = { [P in [1]]: T[P][2] null };

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Akeyof T

B| null

CT

D& null

Attempts:

3 left

💡 Hint

Common Mistakes

Using T instead of keyof T for keys.

Using & null which is intersection, not union.

Use keyof T to iterate keys and | null to add nullability.

5fill in blank

hard

Fill all three blanks to create a mapped type that picks only string properties from a type.

Typescript

type PickStrings<T> = { [[1] in keyof T as T[[2]] extends [3] ? [1] : never]: T[[1]] };

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AP

Cstring

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for keys in the two places.

Using number instead of string in the condition.

This uses key remapping in mapped types to select keys where the property type extends string.