Whitelist and transform options in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable whitelist in ValidationPipe.

NestJS

app.useGlobalPipes(new ValidationPipe({ whitelist: [1] }));

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

Afalse

Btrue

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using false disables whitelist and allows extra properties.

Passing null or undefined causes runtime errors.

Setting whitelist to true removes properties that do not have decorators in DTOs.

2fill in blank

medium

Complete the code to enable automatic transformation in ValidationPipe.

NestJS

app.useGlobalPipes(new ValidationPipe({ transform: [1] }));

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

Atrue

Bfalse

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting transform to false disables automatic conversion.

Passing null or undefined does not enable transformation.

Setting transform to true converts payloads to DTO instances automatically.

3fill in blank

hard

Fix the error in the ValidationPipe options to whitelist and transform correctly.

NestJS

app.useGlobalPipes(new ValidationPipe({ whitelist: [1], transform: [2] }));

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

Afalse

Btrue

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting one option to false disables that feature.

Using null or undefined causes unexpected behavior.

Both whitelist and transform should be set to true to enable filtering and conversion.

4fill in blank

hard

Fill both blanks to configure ValidationPipe with whitelist and transform enabled.

NestJS

app.useGlobalPipes(new ValidationPipe({ whitelist: [1], transform: [2] }));

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

Atrue

Bfalse

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting either option to false disables that feature.

Using null or undefined leads to errors.

Both whitelist and transform must be true to filter unwanted properties and convert payloads.

5fill in blank

hard

Fill all three blanks to configure ValidationPipe with whitelist, transform, and forbidNonWhitelisted options.

NestJS

app.useGlobalPipes(new ValidationPipe({ whitelist: [1], transform: [2], forbidNonWhitelisted: [3] }));

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

Atrue

Bfalse

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Setting forbidNonWhitelisted to true causes validation errors on extra properties.

Using null or undefined for any option causes unexpected behavior.

Enable whitelist and transform by setting them to true. Set forbidNonWhitelisted to false to allow extra properties without error.

Practice

1. What does the whitelist option do when enabled in NestJS validation pipe?

easy

A. It disables validation for unknown properties.

B. It removes any properties not defined in the DTO from the input data.

C. It transforms input data types to match the DTO automatically.

D. It logs all incoming requests for debugging.

Solution

1. Step 1: Understand the whitelist option purpose

   The whitelist option removes any properties from the input that are not explicitly defined in the DTO (Data Transfer Object).

2. Step 2: Compare with other options

   Transform changes types, logging is unrelated, and disabling validation is not whitelist's role.

3. Final Answer:

   It removes any properties not defined in the DTO from the input data. -> Option B

4. Quick Check:

   Whitelist = remove extra fields [OK]

Hint: Whitelist = remove unwanted fields from input [OK]

Common Mistakes:

• Confusing whitelist with transform option
• Thinking whitelist disables validation
• Assuming whitelist logs data

2. Which is the correct way to enable both whitelist and transform options in a global validation pipe in NestJS?

easy

A. app.useGlobalPipes(new ValidationPipe({ whitelist: false, transform: true }));

B. app.useGlobalPipes(new ValidationPipe(whitelist: true, transform: true));

C. app.useGlobalPipes(new ValidationPipe({ whitelist: 'true', transform: 'true' }));

D. app.useGlobalPipes(new ValidationPipe({ whitelist: true, transform: true }));

Solution

1. Step 1: Check the syntax for ValidationPipe options

   Options must be passed as an object with keys and boolean values inside curly braces.

2. Step 2: Validate option values

   Both whitelist and transform must be boolean true, not strings or missing braces.

3. Final Answer:

   app.useGlobalPipes(new ValidationPipe({ whitelist: true, transform: true })); -> Option D

4. Quick Check:

   Correct object syntax with booleans [OK]

Hint: Use object with booleans inside braces for options [OK]

Common Mistakes:

• Missing curly braces around options
• Using strings instead of booleans
• Setting whitelist to false by mistake

3. Given this DTO and input, what will be the output after validation with whitelist: true and transform: true?

class UserDto {
  name: string;
  age: number;
}

const input = { name: 'Alice', age: '30', extra: 'remove me' };

medium

A. { name: 'Alice', age: 30 }

B. { name: 'Alice', age: '30', extra: 'remove me' }

C. { name: 'Alice', age: 30, extra: 'remove me' }

D. Validation error due to extra property

Solution

1. Step 1: Apply whitelist option

   The extra property 'extra' is removed because whitelist is true.

2. Step 2: Apply transform option

   The age string '30' is converted to number 30 as per DTO type.

3. Final Answer:

   { name: 'Alice', age: 30 } -> Option A

4. Quick Check:

   Whitelist removes extra, transform converts types [OK]

Hint: Whitelist removes extras; transform converts types [OK]

Common Mistakes:

• Expecting extra fields to remain
• Not realizing transform converts string to number
• Thinking validation fails on extra fields with whitelist

4. You set whitelist: true but forgot transform: true in your ValidationPipe. What issue might you face when receiving input with number fields as strings?

medium

A. Input number fields remain strings, causing type errors later.

B. Extra fields are not removed from input.

C. ValidationPipe throws a syntax error.

D. Input is automatically converted to correct types.

Solution

1. Step 1: Understand whitelist without transform

   Whitelist removes extra fields but does not convert types.

2. Step 2: Consequence of missing transform

   Number fields sent as strings remain strings, which can cause errors in code expecting numbers.

3. Final Answer:

   Input number fields remain strings, causing type errors later. -> Option A

4. Quick Check:

   Missing transform = no type conversion [OK]

Hint: Whitelist only removes fields; transform converts types [OK]

Common Mistakes:

• Assuming whitelist converts types
• Expecting syntax errors from missing transform
• Thinking extra fields remain with whitelist true

5. You want to accept a nested object in your DTO and ensure unwanted fields are removed and types are converted. Which ValidationPipe options setup correctly handles nested objects with whitelist and transform?

hard

A. new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true })

B. new ValidationPipe({ whitelist: false, transform: true, transformOptions: { enableImplicitConversion: false } })

C. new ValidationPipe({ whitelist: true, transform: true, transformOptions: { enableImplicitConversion: true } })

D. new ValidationPipe({ whitelist: true, transform: false, forbidNonWhitelisted: true })

Solution

1. Step 1: Recognize nested object handling needs transform options

   For nested objects, enabling implicit conversion helps transform nested types correctly.

2. Step 2: Check options for whitelist and transform

   Whitelist true removes unwanted fields; transform true with transformOptions enables nested conversion.

3. Step 3: Compare other options

   Options with forbidNonWhitelisted: true throw errors on extras instead of removing; others disable whitelist or transform or lack transformOptions.

4. Final Answer:

   new ValidationPipe({ whitelist: true, transform: true, transformOptions: { enableImplicitConversion: true } }) -> Option C

5. Quick Check:

   whitelist + transform + enableImplicitConversion [OK]

Hint: Use transformOptions with enableImplicitConversion for nested objects [OK]

Common Mistakes:

• Ignoring transformOptions for nested conversion
• Disabling transform but expecting type conversion
• Not using whitelist to remove extra nested fields