ValidationPipe in depth in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ValidationPipe in NestJS?

ValidationPipe automatically checks incoming data against defined rules to ensure it is correct and safe before it reaches your code.

intermediate

How do you enable automatic transformation of input data types using ValidationPipe?

By setting the option { transform: true } when creating the ValidationPipe, NestJS converts input data to the expected types defined in DTO classes.

beginner

What happens if validation fails when using ValidationPipe?

ValidationPipe throws a detailed error response explaining which fields failed and why, preventing the controller from running with bad data.

intermediate

Explain the difference between whitelist and forbidNonWhitelisted options in ValidationPipe.

Whitelist removes any properties not in the DTO from the input. forbidNonWhitelisted throws an error if extra properties are present, helping catch unwanted data.

beginner

How can you apply ValidationPipe globally in a NestJS application?

Use app.useGlobalPipes(new ValidationPipe()) in the main.ts file to apply validation to all incoming requests automatically.

What option must be set to true to convert input strings to expected types in ValidationPipe?

A transform

B whitelist

C forbidNonWhitelisted

D skipMissingProperties

Setting transform: true enables automatic type conversion of input data.

Which ValidationPipe option removes properties not defined in the DTO?

A forbidNonWhitelisted

B transform

C stopAtFirstError

D whitelist

Whitelist removes extra properties from the input data.

What does ValidationPipe do if input data fails validation?

A Throws an error and stops request processing

B Logs errors but continues

C Ignores errors and passes data

D Automatically fixes errors

ValidationPipe throws an error to prevent bad data from reaching your code.

How do you apply ValidationPipe to all routes in a NestJS app?

A Add ValidationPipe to each controller method

B Set validation in DTO classes only

C Use app.useGlobalPipes(new ValidationPipe()) in main.ts

D Import ValidationPipe in every module

Using app.useGlobalPipes applies ValidationPipe globally.

What does forbidNonWhitelisted option do in ValidationPipe?

A Automatically adds missing properties

B Throws an error if extra properties exist

C Removes extra properties silently

D Ignores validation errors

forbidNonWhitelisted causes validation to fail if unexpected properties are present.

Describe how ValidationPipe helps keep your NestJS app safe and clean.

Think about how ValidationPipe acts like a gatekeeper for incoming data.

Explain the difference between using whitelist and forbidNonWhitelisted options in ValidationPipe.

One quietly cleans input, the other loudly complains.

Practice

1. What is the main purpose of ValidationPipe in a NestJS application?

easy

A. To check and clean incoming data before it is used

B. To handle database connections automatically

C. To manage user authentication and sessions

D. To serve static files like images and CSS

Solution

1. Step 1: Understand ValidationPipe role

   ValidationPipe is designed to validate and sanitize incoming request data to ensure it meets expected rules.

2. Step 2: Compare with other options

   Options B, C, and D relate to other parts of NestJS or web apps, not data validation.

3. Final Answer:

   To check and clean incoming data before it is used -> Option A

4. Quick Check:

   ValidationPipe purpose = data validation [OK]

Hint: ValidationPipe = data check and clean before use [OK]

Common Mistakes:

• Confusing ValidationPipe with authentication or database handling
• Thinking ValidationPipe serves static files
• Assuming ValidationPipe manages sessions

2. Which of the following is the correct way to enable automatic transformation of input data types using ValidationPipe in NestJS?

easy

A. new ValidationPipe({ forbidNonWhitelisted: true })

B. new ValidationPipe({ whitelist: true })

C. new ValidationPipe({ transform: true })

D. new ValidationPipe({ skipMissingProperties: true })

Solution

1. Step 1: Identify transform option

   The transform: true option enables automatic conversion of input data types to expected types.

2. Step 2: Understand other options

   whitelist removes unwanted properties, forbidNonWhitelisted throws error on extra props, skipMissingProperties skips validation for missing props.

3. Final Answer:

   new ValidationPipe({ transform: true }) -> Option C

4. Quick Check:

   transform option enables type conversion [OK]

Hint: Use transform: true to convert input types automatically [OK]

Common Mistakes:

• Confusing whitelist with transform
• Using forbidNonWhitelisted to transform data
• Thinking skipMissingProperties enables transformation

3. Given this DTO and ValidationPipe setup, what will be the output when sending { "age": "25" } in the request body?

class UserDto {
  @IsInt()
  age: number;
}

// In controller
@Post()
create(@Body(new ValidationPipe({ transform: true })) user: UserDto) {
  return typeof user.age;
}

medium

A. "string"

B. "undefined"

C. Validation error thrown

D. "number"

Solution

1. Step 1: Check ValidationPipe transform option

   Transform is true, so input string "25" will be converted to number 25.

2. Step 2: Determine returned type

   Since age is transformed to number, typeof user.age returns "number".

3. Final Answer:

   "number" -> Option D

4. Quick Check:

   transform converts string to number, so typeof = "number" [OK]

Hint: transform: true converts strings to numbers automatically [OK]

Common Mistakes:

• Assuming input stays string without transform
• Expecting validation error for string number
• Thinking age is undefined without explicit conversion

4. Identify the error in this ValidationPipe usage that causes unexpected behavior:

@Post()
create(@Body(new ValidationPipe({ whitelist: true })) user: UserDto) {
  return user;
}

// UserDto has properties: name (string), age (number)

Request body: { "name": "Alice", "age": 30, "extra": "value" }

medium

A. The ValidationPipe should have forbidNonWhitelisted to throw error on extra props

B. The whitelist option removes the extra property silently

C. The ValidationPipe is missing transform: true to convert types

D. The extra property causes a validation error

Solution

1. Step 1: Understand whitelist behavior

   Whitelist removes properties not in DTO silently without error.

2. Step 2: Check how to throw error on extra props

   To throw error on extra properties, forbidNonWhitelisted: true must be set.

3. Final Answer:

   The ValidationPipe should have forbidNonWhitelisted to throw error on extra props -> Option A

4. Quick Check:

   forbidNonWhitelisted throws error on extra props [OK]

Hint: Use forbidNonWhitelisted to reject extra properties with error [OK]

Common Mistakes:

• Thinking whitelist throws error by default
• Ignoring need for forbidNonWhitelisted option
• Confusing whitelist with transform option

5. You want to ensure that your NestJS API only accepts properties defined in your DTO, automatically converts input types, and throws an error if extra properties are sent. Which ValidationPipe configuration achieves this?

hard

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

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

C. new ValidationPipe({ transform: true, skipMissingProperties: true })

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

Solution

1. Step 1: Identify needed options

   Whitelist removes extra props, transform converts types, forbidNonWhitelisted throws error on extra props.

2. Step 2: Match options to requirements

   Only new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true }) includes all three: whitelist, transform, and forbidNonWhitelisted.

3. Final Answer:

   new ValidationPipe({ whitelist: true, transform: true, forbidNonWhitelisted: true }) -> Option B

4. Quick Check:

   Whitelist + transform + forbidNonWhitelisted = strict validation [OK]

Hint: Combine whitelist, transform, forbidNonWhitelisted for strict validation [OK]

Common Mistakes:

• Omitting forbidNonWhitelisted to throw errors
• Forgetting transform for type conversion
• Using skipMissingProperties instead of forbidNonWhitelisted