Removing modifiers with minus in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of removing readonly modifier with minus

What is the type of MutablePerson after removing the readonly modifier from Person?

Typescript

type Person = {
  readonly name: string;
  readonly age: number;
};

type MutablePerson = {
  -readonly [K in keyof Person]: Person[K];
};

const p: MutablePerson = { name: "Alice", age: 30 };
p.name = "Bob";

console.log(p.name);

A Compilation error

B Alice

C TypeError at runtime

D Bob

💡 Hint

Removing the readonly modifier allows properties to be changed.

The minus sign before readonly removes the readonly modifier, so the property 'name' can be reassigned. The console.log prints the updated value 'Bob'.

❓ Predict Output

intermediate

Effect of removing optional modifier with minus

What is the type of RequiredUser after removing the optional modifier from User?

Typescript

type User = {
  id: number;
  name?: string;
};

type RequiredUser = {
  -? [K in keyof User]: User[K];
};

const user: RequiredUser = { id: 1, name: "John" };
console.log(user.name);

A John

B Compilation error: Property 'name' is missing

C undefined

D TypeError at runtime

💡 Hint

Removing the optional modifier makes the property required.

The minus sign before ? removes the optional modifier, so 'name' must be present. The code assigns 'John' and logs it.

🔧 Debug

advanced

Why does this code cause a compilation error?

Consider this code snippet using minus modifiers. Which option explains the cause of the error?

Typescript

type Flags = {
  readonly active?: boolean;
};

type MutableFlags = {
  -readonly -? [K in keyof Flags]: Flags[K];
};

const flags: MutableFlags = {};
console.log(flags.active);

A Minus modifiers only work on methods, not properties

B Cannot remove both readonly and optional modifiers at once

C Removing optional modifier requires property to be present, but object is empty

D TypeScript does not support mapped types with multiple modifiers

💡 Hint

Check if the object satisfies the required properties after removing optional.

Removing the optional modifier makes 'active' required, but the object is empty, causing a compilation error.

🧠 Conceptual

advanced

Understanding combined minus modifiers in mapped types

What is the effect of the following mapped type?

type T = { readonly a?: number };
type U = { -readonly -? [K in keyof T]: T[K] };

A Properties in U are mutable and required

B Properties in U are readonly and optional

C Properties in U are mutable and optional

D Properties in U are readonly and required

💡 Hint

Minus removes modifiers from properties.

The minus signs remove readonly and optional modifiers, so properties become mutable and required.

📝 Syntax

expert

Which option causes a syntax error when removing modifiers?

Which of the following mapped type definitions is syntactically invalid in TypeScript when trying to remove modifiers?

A type A = { -readonly [K in keyof T]: T[K] };

B type D = { -readonly? [K in keyof T]: T[K] };

C type C = { -readonly -? [K in keyof T]: T[K] };

D type B = { -? [K in keyof T]: T[K] };

💡 Hint

Check the correct syntax for removing modifiers with minus.

Option B is invalid because '-readonly?' is not valid syntax; modifiers must be removed separately with '-' before each modifier.