Why conditional types are needed in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Typescript

type CheckType<T> = T extends [1] ? 'string' : 'number';

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Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' instead of 'string' in the condition.

The conditional type checks if T extends string. If yes, it returns 'string', else 'number'.

2fill in blank

medium

Complete the code to create a type that returns 'true' if T is assignable to number, otherwise 'false'.

Typescript

type IsNumber<T> = T extends [1] ? true : false;

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Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'number' in the condition.

The conditional type checks if T extends number. If yes, it returns true, else false.

3fill in blank

hard

Fix the error in the conditional type that should return 'Array' if T is an array, else 'NotArray'.

Typescript

type CheckArray<T> = T extends [1] ? 'Array' : 'NotArray';

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AArray

B[]

Cany[]

DArray<any>

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Array' without generic type causes error.

To check if T is an array type, use any[] in the conditional type.

4fill in blank

hard

Fill both blanks to create a conditional type that returns the element type if T is an array, otherwise T itself.

Typescript

type ElementType<T> = T extends [1] ? [2] : T;

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Aany[]

BT[number]

CArray<T>

DT[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array instead of any[] in condition.

The type checks if T is an array (any[]). If yes, it returns the element type using T[number], else returns T.

5fill in blank

hard

Fill all three blanks to create a conditional type that returns 'string' if T is string, 'number' if T is number, otherwise 'unknown'.

Typescript

type TypeName<T> = T extends [1] ? 'string' : T extends [2] ? 'number' : [3];

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Astring

Bnumber

C'unknown'

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using any instead of 'unknown' in the else case.

This conditional type checks if T is string, then returns 'string'. Else if T is number, returns 'number'. Otherwise, returns 'unknown'.