Generic conditional constraints in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic function that accepts a parameter of any type.

Typescript

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

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

Aparam

Bvalue

Citem

Darg

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name different from the one returned causes an error.

Forgetting to specify the parameter type T.

The generic function uses a parameter named 'param' of type T to return the same value.

2fill in blank

medium

Complete the code to constrain the generic type to only types that have a 'length' property.

Typescript

function logLength<T extends [1]>(item: T): number {
  return item.length;
}

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

A{ length: number }

Bstring

Cnumber

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as constraint limits to string only, not all types with length.

Using 'any' removes the constraint.

The generic type T is constrained to types that have a 'length' property of type number.

3fill in blank

hard

Fix the error in the generic function that requires T to have a 'toString' method.

Typescript

function stringify<T extends [1]>(value: T): string {
  return value.toString();
}

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

Aobject

Bstring

C{ toString: () => string }

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'object' does not guarantee 'toString' method signature.

Using 'string' as constraint is incorrect here.

The constraint requires T to have a 'toString' method returning a string.

4fill in blank

hard

Fill both blanks to create a generic function that accepts an object with a key K and returns its value.

Typescript

function getProperty<T, K extends keyof T>(obj: T, key: [1]): T[[2]] {
  return obj[key];
}

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

AK

Bstring

Ckeyof T

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'keyof T' directly in the parameter type causes errors.

Mismatching the key type and return type.

The key parameter is of type K which extends keys of T, and the return type is the value type at key K.

5fill in blank

hard

Fill all three blanks to define a generic function that returns the length of a property K of object T, where that property has a length.

Typescript

function getLength<T, K extends keyof T>(obj: T, key: K): number {
  const value = obj[[1]];
  if (value && typeof value === [2] && 'length' in value) {
    return value.length[3];
  }
  return 0;
}

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

Akey

B'object'

C?? 0

D'string'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'object' in typeof check.

Not using nullish coalescing operator causes runtime errors if length is undefined.

The function accesses obj[key], checks if it's an object with a length property, and returns length or 0 if not present.