Nested conditional types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a nested conditional type that returns 'string' if T is 'number', otherwise 'boolean'.

Typescript

type NestedType<T> = T extends number ? [1] : boolean;

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

Anumber

Bany

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' instead of 'string' as the true branch.

Confusing the order of true and false branches.

The nested conditional type returns string when T extends number, otherwise boolean.

2fill in blank

medium

Complete the nested conditional type to return 'true' if T is 'string', else check if T is 'boolean' and return 'false', otherwise 'never'.

Typescript

type CheckType<T> = T extends string ? true : T extends boolean ? [1] : never;

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

Afalse

Btrue

Cnever

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'true' instead of 'false' for the boolean check.

Using 'never' in the wrong branch.

The nested conditional checks if T is string returning true, else if T is boolean returning false, otherwise never.

3fill in blank

hard

Fix the error in the nested conditional type to correctly return 'number' if T is 'string', else 'string' if T is 'number', otherwise 'boolean'.

Typescript

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

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

Astring

Bnumber

Cany

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'string' instead of 'number' for the first branch.

Mixing up the order of types.

The type should return number when T is string, so the first true branch is number.

4fill in blank

hard

Fill both blanks to create a nested conditional type that returns 'true' if T is 'string', 'false' if T is 'number', otherwise 'never'.

Typescript

type BooleanCheck<T> = T extends string ? [1] : T extends number ? [2] : never;

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

Atrue

Bfalse

Cnever

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping true and false values.

Using 'never' in the wrong place.

The type returns true for string, false for number, and never otherwise.

5fill in blank

hard

Fill all three blanks to define a nested conditional type that returns the uppercase string of T if T is 'string', returns T if T is 'number', otherwise returns 'unknown'.

Typescript

type Transform<T> = T extends string ? [1] : T extends number ? [2] : [3];

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AUppercase<T>

BT

Cunknown

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Not using Uppercase for strings.

Returning wrong types for number or default case.

The type uses Uppercase<T> for strings, returns T for numbers, and unknown otherwise.