How TypeScript infers generic types - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a generic function that returns the input value.

Typescript

function identity<T>([1]: T): T {
  return [1];
}

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

Avalue

Btype

Cinput

Darg

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that doesn't match the return statement.

Using a non-generic type.

The generic function uses a parameter named 'value' of type T and returns it.

2fill in blank

medium

Complete the code to call the generic function with a string argument and let TypeScript infer the type.

Typescript

const result = identity([1]);

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

A42

Bnull

Ctrue

D'hello'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a number or boolean instead of a string.

Passing null which is not inferred as a string.

Passing a string literal 'hello' lets TypeScript infer T as string.

3fill in blank

hard

Fix the error in the generic function call by specifying the type explicitly.

Typescript

const result = identity<[1]>(42);

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

Aboolean

Bstring

Cnumber

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Specifying a type that doesn't match the argument type.

Omitting the generic type when explicit specification is needed.

Explicitly specifying 'number' as the generic type matches the argument 42.

4fill in blank

hard

Fill both blanks to define a generic function that returns the length of an array of generic type T.

Typescript

function getLength<[1]>(arr: [2][]): number {
  return arr.length;
}

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

AT

BU

DV

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic type names for the parameter and array element.

Not using a generic type for the array elements.

The generic type parameter is T, and the array parameter is of type T[].

5fill in blank

hard

Fill all three blanks to create a generic function that returns the first element of an array of generic type T or undefined if empty.

Typescript

function firstElement<[1]>(arr: [2][]): [3] | undefined {
  return arr.length > 0 ? arr[0] : undefined;
}

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

AT

BU

DV

Attempts:

3 left

💡 Hint

Common Mistakes

Using different generic type names for parameters and return type.

Not including undefined in the return type.

The generic type parameter is T, the array is of type T[], and the return type is T or undefined.