Default generic types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a generic function with a default type.

Typescript

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

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Aboolean

Bnumber

Cany

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a specific type like string or number as default when flexibility is needed.

The default generic type is any, so if no type is specified, T defaults to any.

2fill in blank

medium

Complete the code to create a generic class with a default type.

Typescript

class Box<T = [1]> {
  contents: T;
  constructor(value: T) {
    this.contents = value;
  }
}

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Astring

Bnumber

Cboolean

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean when the example expects string.

The default generic type is string, so if no type is specified, T defaults to string.

3fill in blank

hard

Fix the error in the generic function by adding a default type.

Typescript

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

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A extends string

B extends number

C = number

D = string

Attempts:

3 left

💡 Hint

Common Mistakes

Using extends instead of = for default types.

Adding = string sets the default generic type to string, fixing the error if no type is provided.

4fill in blank

hard

Fill both blanks to create a generic interface with a default type and a property using that type.

Typescript

interface Container<T = [1]> {
  value: [2];
}

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Anumber

Bstring

CT

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using the property type as a concrete type instead of the generic parameter.

The default generic type is string, and the property value uses the generic type T.

5fill in blank

hard

Fill all three blanks to define a generic function with two type parameters, one with a default, and return a tuple.

Typescript

function pair<T, U = [1]>(first: T, second: U): [[2], [3]] {
  return [first, second];
}

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

Astring

BT

CU

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of generic parameters in the return type.

The second generic type U defaults to number. The return tuple uses types T and U.