Excess property checks vs structural compatibility in Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an object with a required property 'name'.

Typescript

interface Person { name: string; }
const person: Person = { name: [1] };

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

A"Alice"

BAlice

Cname

D123

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around string values.

Using a variable name instead of a string literal.

The property 'name' must be a string, so we use a string literal like "Alice".

2fill in blank

medium

Complete the code to assign an object with an extra property 'age' to a variable of type Person.

Typescript

interface Person { name: string; }
const person: Person = { name: "Bob", [1]: 30 };

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

Aheight

Baddress

Cage

Dweight

Attempts:

3 left

💡 Hint

Common Mistakes

Using a property name that is in the interface (like 'name').

Using a property name unrelated to the example.

The extra property 'age' is not declared in Person, so TypeScript will check for excess properties.

3fill in blank

hard

Fix the error in the code by completing the type assertion to allow extra properties.

Typescript

interface Person { name: string; }
const p: Person = { name: "Carol", age: 25 } as [1];

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

Aany

BPerson

Cunknown

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Person' which does not allow extra properties.

Using 'unknown' which requires further checks.

Using 'any' in a type assertion disables excess property checks, allowing extra properties.

4fill in blank

hard

Fill both blanks to create a function that accepts an object with at least a 'name' property and any extra properties.

Typescript

function greet(person: [1] & [2]) {
  console.log(`Hello, ${person.name}`);
}

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

A{ name: string }

BRecord<string, unknown>

Cany

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'any' which disables type safety.

Using 'object' which does not specify properties.

The intersection of a specific type and a Record allows extra properties while requiring 'name'.

5fill in blank

hard

Fill all three blanks to create a variable with structural compatibility allowing extra properties without errors.

Typescript

interface Person { name: string; }
const p: Person = { name: "Dave", [1]: 40, [2]: "blue" } as [3];

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

Aage

BfavoriteColor

Cany

DPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Person' in the assertion which causes errors due to extra properties.

Using property names not matching the example.

Using 'any' as a type assertion allows assigning an object with extra properties to Person without errors.

Practice

1. What does TypeScript's excess property checks primarily prevent when assigning an object literal to a variable of a specific type?

easy

A. Assigning object literals with extra properties not defined in the target type

B. Assigning objects with missing properties

C. Assigning objects with incompatible property types

D. Assigning objects with properties in a different order

Solution

1. Step 1: Understand excess property checks context

   Excess property checks happen when assigning an object literal directly to a typed variable.

2. Step 2: Identify what excess property checks prevent

   They prevent extra properties not declared in the target type from being assigned, catching typos or mistakes.

3. Final Answer:

   Assigning object literals with extra properties not defined in the target type -> Option A

4. Quick Check:

   Excess property checks = prevent extra properties [OK]

Hint: Excess checks block extra properties in direct object literals [OK]

Common Mistakes:

• Confusing excess property checks with missing property errors
• Thinking property order matters
• Assuming excess checks apply to variables, not literals

2. Which of the following TypeScript assignments will cause an excess property check error?

easy

A. const obj: {name: string} = {name: 'Dave'} as {name: string};

B. const extra = {name: 'Bob', age: 25}; const obj: {name: string} = extra;

C. const obj: {name: string} = {name: 'Carol'};

D. const obj: {name: string} = {name: 'Alice', age: 30};

Solution

1. Step 1: Check direct object literal assignments

   const obj: {name: string} = {name: 'Alice', age: 30}; assigns a literal with an extra property age directly to a typed variable, triggering excess property checks.

2. Step 2: Compare with other options

   const extra = {name: 'Bob', age: 25}; const obj: {name: string} = extra; assigns from a variable, so excess property checks do not apply. Options C and D have no extra properties.

3. Final Answer:

   const obj: {name: string} = {name: 'Alice', age: 30}; -> Option D

4. Quick Check:

   Direct literal with extra property = error [OK]

Hint: Extra properties in direct literals cause errors, variables do not [OK]

Common Mistakes:

• Thinking variables cause excess property errors
• Ignoring the difference between literals and variables
• Assuming type assertions always cause errors

3. What will be the output of this TypeScript code?

interface Person { name: string; }
const extra = { name: 'Eve', age: 40 };
const p: Person = extra;
console.log(p.name);

medium

A. Runtime error

B. Error: Excess property 'age' not allowed

C. Eve

D. undefined

Solution

1. Step 1: Understand assignment from variable

   Assigning from variable extra bypasses excess property checks, so assignment is allowed.

2. Step 2: Check console output

   Accessing p.name prints 'Eve' as expected.

3. Final Answer:

   Eve -> Option C

4. Quick Check:

   Variable assignment allows extra properties, prints name [OK]

Hint: Variables with extra props assign fine; literals with extras error [OK]

Common Mistakes:

• Expecting excess property error on variable assignment
• Confusing compile-time error with runtime output
• Assuming extra properties are removed

4. Identify the error in this TypeScript code and fix it:

interface Car { make: string; }
const car = { make: 'Toyota', year: 2020 };
const myCar: Car = { make: 'Honda', year: 2018 };

medium

A. Error due to extra property 'year' in direct object literal; fix by assigning from variable

B. Error due to missing property 'year'; fix by adding 'year' to interface

C. No error; code is correct

D. Error due to wrong property type; fix by changing 'year' to string

Solution

1. Step 1: Identify excess property check error

   Direct assignment of object literal with extra property year causes error.

2. Step 2: Fix by assigning from variable

   Assigning from variable car avoids excess property checks and fixes error.

3. Final Answer:

   Error due to extra property 'year' in direct object literal; fix by assigning from variable -> Option A

4. Quick Check:

   Direct literal with extra prop = error; use variable [OK]

Hint: Assign extra props via variable, not direct literal [OK]

Common Mistakes:

• Adding extra properties to interface unnecessarily
• Ignoring excess property check rules
• Assuming no error on direct literal with extra props

5. Given this interface and code, which assignment will NOT cause a TypeScript error?

interface User { username: string; }

// Option 1
const u1: User = { username: 'john', email: '[e-mail]' };

// Option 2
const extraUser = { username: 'jane', email: '[e-mail]' };
const u2: User = extraUser;

// Option 3
const u3 = { username: 'doe', email: '[e-mail]' } as User;

// Option 4
const u4: User = { username: 'max' };

hard

A. Only Option 3 and Option 4

B. Only Option 2 and Option 4

C. Only Option 1 and Option 3

D. All options cause errors

Solution

1. Step 1: Analyze Option 1

   Direct literal with extra property email causes excess property check error.

2. Step 2: Analyze Option 2 and 4

   Option 2 assigns from variable, so no error. Option 4 matches interface exactly, no error.

3. Step 3: Analyze Option 3

   Type assertion on direct object literal with extra property email still triggers excess property checks, causing an error.

4. Final Answer:

   Only Option 2 and Option 4 -> Option B

5. Quick Check:

   Direct literal with extras errors; variable assignment and exact match okay [OK]

Hint: Direct literals with extras error; variables and exact matches okay [OK]

Common Mistakes:

• Thinking type assertions bypass excess property checks on object literals
• Thinking all direct literals with extras are allowed
• Ignoring structural compatibility rules