Conditional type with generics in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a generic type that returns string if the type is number, otherwise boolean.

Typescript

type CheckType<T> = T extends [1] ? string : boolean;

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Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of number in the condition.

Confusing the true and false branches of the conditional type.

The conditional type checks if T extends number. If yes, it returns string, else boolean.

2fill in blank

medium

Complete the code to create a conditional type that returns T if it extends string, otherwise returns never.

Typescript

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

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Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or other types instead of string.

Returning the wrong type in the true branch.

This conditional type returns T only if it extends string. Otherwise, it returns never, which means no valid type.

3fill in blank

hard

Fix the error in the conditional type that should return Array<T> if T extends string, else T.

Typescript

type WrapInArray<T> = T extends [1] ? Array<T> : T;

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Aany

Bstring

Cboolean

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or other types in the extends clause.

Confusing the true and false branches.

The conditional type checks if T extends string. If yes, it wraps T in an array, else returns T as is.

4fill in blank

hard

Fill the blank to create a conditional type that returns Promise<T> if T extends string, otherwise returns T.

Typescript

type AsyncType<T> = T [1] string ? Promise<T> : T;

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A==

Bimplements

Cextends=

Dextends

Attempts:

3 left

💡 Hint

Common Mistakes

Using == or extends= which are invalid in this context.

Confusing the order of keywords.

The correct syntax for conditional types uses extends to check if T is a subtype of string.

5fill in blank

hard

Fill all three blanks to define a conditional type that returns string if T extends number, returns boolean if T extends string, otherwise returns never.

Typescript

type MultiCheck<T> = T [1] number ? string : T [2] string ? boolean : [3];

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Aextends

Bimplements

Cnever

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using implements instead of extends.

Using any instead of never as fallback.

This nested conditional type checks if T extends number, then returns string. Otherwise, it checks if T extends string, then returns boolean. If neither, it returns never.