How do you validate arrays and conditional fields with class-validator in NestJS?

Q: How do you validate arrays and conditional fields with class-validator in NestJS?

// Array validation export class CreateOrderDto { @IsArray() @ArrayMinSize(1) @ArrayMaxSize(50) @IsUUID('4', { each: true }) // each: true applies the rule per item productIds: string[]; } // Conditional validation export class PaymentDto { @IsEnum(['card', 'paypal']) method: string; // Required only when method === 'card' @ValidateIf(o => o.method === 'card') @IsCreditCard() cardNumber?: string; // Required only when method === 'paypal' @ValidateIf(o => o.method === 'paypal') @IsEmail() paypalEmail?: string; } each: true — applies the decorator to every element in the array individually. @IsArray() — validates the property is an array before per-item checks run. @ArrayMinSize(n) / @ArrayMaxSize(n) — length constraints on the array. @ValidateIf(condition) — the validator is entirely skipped when condition returns false. Combine @ValidateIf with @IsOptional when the field may also be absent entirely.

Difficulty: 5/10

array validation, conditional validation, class-validator decorators

Use the each: true option on any decorator to apply it per array item. Use @ValidateIf(condition) for conditional validation — the decorator is entirely skipped when the condition returns false, so no error is thrown for undefined optional conditional fields.

Array validation and conditional fields

Array and conditional validation rules:

1
each: true — applies the decorator to every element in the array individually.
2
@IsArray() — validates the property is an array before per-item checks run.
3
@ArrayMinSize(n) / @ArrayMaxSize(n) — length constraints on the array.
4
@ValidateIf(condition) — the validator is entirely skipped when condition returns false.
5
Combine @ValidateIf with @IsOptional when the field may also be absent entirely.

Scenario Questions

0-2 years experience

2-5 years experience

1You need to accept a request where `items` is an array of objects, each requiring a positive `price`, and the array must contain at most 10 elements. The client sometimes sends an empty array, which should be rejected. Walk me through your validation implementation and any pitfalls.

Follow-up Questions