Inferring types with infer keyword in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to infer the type of the first element in a tuple.

Typescript

type First<T extends any[]> = T extends [[1], ...any[]] ? [1] : never;

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Ainfer R

Binfer T

Cinfer U

Dinfer V

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type instead of infer.

Not using the spread operator ...any[] to match the rest of the tuple.

The infer keyword is used to declare a type variable U that captures the first element's type in the tuple T.

2fill in blank

medium

Complete the code to infer the return type of a function type.

Typescript

type ReturnType<T> = T extends (...args: any[]) => [1] ? [1] : never;

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Ainfer R

Bvoid

Cany

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed type like void instead of infer.

Not matching the function signature correctly.

The infer R captures the return type of the function T.

3fill in blank

hard

Fix the error in the code to infer the element type of an array type.

Typescript

type ElementType<T> = T extends Array<[1]> ? [1] : never;

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AE

Binfer E

Cinfer T

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using a plain type variable without infer.

Using any which loses type information.

infer E is needed to capture the element type inside the array type T.

4fill in blank

hard

Fill both blanks to infer the argument types of a function type as a tuple.

Typescript

type Args<T> = T extends (...args: [1]) => any ? [2] : never;

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Ainfer A

Bany[]

Cinfer R

Dunknown[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed type like any[] instead of infer.

Using different type variables in the two blanks.

infer A captures the tuple of argument types from the function type T.

5fill in blank

hard

Fill all three blanks to infer the resolved type from a Promise type.

Typescript

type UnwrapPromise<T> = T extends Promise<[1]> ? [1] : T extends Promise<[2]> ? [3] : T;

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Ainfer U

Bany

CU

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using different type variables for the two Promise checks.

Not using infer to capture the inner type.

The first infer U captures the type inside the Promise. Then U is used to return the resolved type.