Satisfies operator in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use the satisfies operator to check the type of the object.

Typescript

const user = { name: "Alice", age: 30 } [1] { name: string; age: number };

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

Asatisfies

B:

Cas

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':' instead of 'satisfies' which only declares type but does not check compatibility.

Using 'as' which changes the type instead of checking it.

The satisfies operator is used to check that the object matches the given type without changing the type of the object.

2fill in blank

medium

Complete the code to ensure the object matches the interface using the satisfies operator.

Typescript

interface Product { id: number; name: string; }
const item = { id: 1, name: "Book" } [1] Product;

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

Aimplements

Bextends

Cas

Dsatisfies

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' which is only valid in class declarations.

Using 'as' which casts the type instead of checking compatibility.

The satisfies operator ensures that 'item' matches the 'Product' interface without changing its type.

3fill in blank

hard

Fix the error in the code by using the correct operator to check the type compatibility.

Typescript

const config = { debug: true, version: "1.0" } [1] { debug: boolean; version: string };

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

A:

Bsatisfies

Cimplements

Das

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':' which only declares type but does not check compatibility.

Using 'as' which casts the type and may hide errors.

Using 'satisfies' checks that 'config' matches the type without changing its type, fixing the error.

4fill in blank

hard

Fill both blanks to create a typed object that satisfies the interface without changing its type.

Typescript

interface User { id: number; username: string; }
const newUser = [1] [2] User;

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

A{ id: 5, username: "john" }

Bas

Csatisfies

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':' instead of 'satisfies' which only declares type.

Using 'as' which casts the type and changes it.

The object literal is assigned and checked with 'satisfies' to ensure it matches the User interface without casting.

5fill in blank

hard

Fill all three blanks to create a constant object that satisfies the type and access its property safely.

Typescript

type Settings = { theme: string; notifications: boolean };
const settings = [1] [2] Settings;
const theme = settings.[3];

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

A{ theme: "dark", notifications: true }

Bsatisfies

Ctheme

Das

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' instead of 'satisfies' which casts the type.

Accessing a property name that does not exist.

The object literal is checked with 'satisfies' to ensure it matches the Settings type, then the 'theme' property is accessed.

Practice

1.

What does the satisfies operator do in TypeScript?

easy

A. Creates a new type from existing types

B. Converts a value to a different type

C. Removes properties from an object

D. Checks if a value matches a type without changing the value

Solution

1. Step 1: Understand the purpose of satisfies

   The satisfies operator checks if a value fits a type but keeps the original value unchanged.

2. Step 2: Compare with other type operations

   Unlike type conversion or creating new types, satisfies only verifies compatibility without altering the value.

3. Final Answer:

   Checks if a value matches a type without changing the value -> Option D

4. Quick Check:

   satisfies = type check only [OK]

Hint: Remember: satisfies checks type but keeps original value [OK]

Common Mistakes:

• Thinking it converts or changes the value
• Confusing it with type casting
• Assuming it removes object properties

2.

Which of the following is the correct syntax to use the satisfies operator in TypeScript?

const data = { name: "Alice", age: 30 } ___ Person;

easy

A. :

B. as

C. satisfies

D. implements

Solution

1. Step 1: Recall the syntax of satisfies

   The satisfies operator is used between the value and the type like: value satisfies Type.

2. Step 2: Identify the correct operator in the code

   In the example, the blank should be filled with satisfies to check the type without changing data.

3. Final Answer:

   satisfies -> Option C

4. Quick Check:

   Syntax uses satisfies keyword [OK]

Hint: Use 'satisfies' keyword between value and type [OK]

Common Mistakes:

• Using ':' which is for type annotations
• Using 'as' which is for type assertions
• Using 'implements' which is for classes

3.

What will be the output of this TypeScript code?

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

const user = {
  name: "Bob",
  age: 25,
  city: "Paris"
} satisfies Person;

console.log(user.city);

medium

A. "Paris"

B. Error: Property 'city' does not exist on type 'Person'

C. undefined

D. null

Solution

1. Step 1: Understand what satisfies does to the object

   The satisfies operator checks that user matches Person type but keeps all original properties, including city.

2. Step 2: Check the console.log output

   Since user still has city, logging user.city prints "Paris" without error.

3. Final Answer:

   "Paris" -> Option A

4. Quick Check:

   satisfies keeps extra properties [OK]

Hint: Extra properties stay accessible with satisfies [OK]

Common Mistakes:

• Assuming satisfies removes extra properties
• Expecting a type error on extra properties
• Confusing satisfies with type assertion

4.

Identify the error in this TypeScript code using satisfies:

type Config = { port: number; host: string };

const serverConfig = {
  port: "8080",
  host: "localhost"
} satisfies Config;

medium

A. Type error: 'port' should be a number, not a string

B. No error, code is correct

C. Syntax error: 'satisfies' cannot be used here

D. Runtime error: 'host' is missing

Solution

1. Step 1: Check the type of each property in serverConfig

   The port property is a string "8080" but Config expects a number.

2. Step 2: Understand what satisfies checks

   satisfies verifies type compatibility and will raise a type error because of the wrong type for port.

3. Final Answer:

   Type error: 'port' should be a number, not a string -> Option A

4. Quick Check:

   Type mismatch causes error with satisfies [OK]

Hint: Check property types carefully with satisfies [OK]

Common Mistakes:

• Ignoring type mismatch on properties
• Thinking satisfies changes the value type
• Assuming no error if extra properties exist

5.

You want to create a constant object settings that must have theme as a string and notifications as a boolean. You also want to keep extra properties without losing their types. Which code correctly uses satisfies to achieve this?

hard

A. const settings = { theme: "dark", notifications: true, version: 2 } as { theme: string; notifications: boolean };

B. const settings = { theme: "dark", notifications: true, version: 2 } satisfies { theme: string; notifications: boolean };

C. const settings: { theme: string; notifications: boolean } = { theme: "dark", notifications: true, version: 2 };

D. const settings = { theme: "dark", notifications: true, version: 2 };

Solution

1. Step 1: Understand the goal

   You want settings to be checked against a type but keep extra properties like version with their types intact.

2. Step 2: Compare options

   const settings = { theme: "dark", notifications: true, version: 2 } satisfies { theme: string; notifications: boolean }; uses satisfies which checks the required properties but keeps extra ones with full types. const settings: { theme: string; notifications: boolean } = { theme: "dark", notifications: true, version: 2 }; uses type annotation which drops extra properties. const settings = { theme: "dark", notifications: true, version: 2 } as { theme: string; notifications: boolean }; uses type assertion which forces the type and may lose extra property types. const settings = { theme: "dark", notifications: true, version: 2 }; has no type checking.

3. Final Answer:

   const settings = { theme: "dark", notifications: true, version: 2 } satisfies { theme: string; notifications: boolean }; -> Option B

4. Quick Check:

   satisfies keeps extra properties typed [OK]

Hint: Use satisfies to keep extra properties typed safely [OK]

Common Mistakes:

• Using type annotation and losing extra properties
• Using type assertion and ignoring type safety
• Not checking required properties at all