Non-distributive conditional types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a conditional type that checks if T is a string.

Typescript

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

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

Astring

Bnumber

Cany

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' instead of 'string' in the extends clause.

Confusing the syntax of conditional types.

The conditional type checks if T extends string. If yes, it returns true, otherwise false.

2fill in blank

medium

Complete the code to prevent distributive behavior by wrapping T in a tuple.

Typescript

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

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

Anumber

Bany

Cstring

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Not wrapping T in a tuple, causing distributive behavior.

Using the wrong type inside the extends clause.

Wrapping T in a tuple [T] stops TypeScript from distributing the conditional type over unions.

3fill in blank

hard

Fix the error in the conditional type to avoid distribution over union types.

Typescript

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

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

Aboolean

Bstring

Cnumber

D[string]

Attempts:

3 left

💡 Hint

Common Mistakes

Using string directly, which causes distribution.

Not understanding how wrapping in tuples affects conditional types.

Using [string] inside the extends clause wraps the type in a tuple, preventing distribution.

4fill in blank

hard

Fill both blanks to create a non-distributive conditional type that checks if T is assignable to U.

Typescript

type IsAssignable<T, U> = [T] extends [[1]] ? [2] : false;

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

AU

Btrue

CT

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Not wrapping T in a tuple.

Returning T instead of true in the true branch.

This type checks if T is assignable to U without distribution by wrapping T in a tuple. If yes, it returns true, else false.

5fill in blank

hard

Fill all three blanks to create a non-distributive conditional type that returns the type if it extends U, else never.

Typescript

type FilterType<T, U> = [T] extends [[1]] ? [2] : [3];

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

AU

BT

Cnever

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Returning unknown instead of never in the false branch.

Not wrapping T in a tuple.

This type filters T by checking if it extends U without distribution. If yes, it returns T, else never.