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In the nest.js application on controller level I have to validate DTO.

I've faced with difficulty to check if item is not null (request should be rejected if any list item is null or undefined )

Code bellow demonstrates my configured verifications.

import { ArrayMinSize, IsArray } from 'class-validator'
export class ReminderPayload {
    // ...
    @IsArray()
    @ArrayMinSize(1)
    recipients: string[]
Question
I'm looking for help to reject requests with body data like
    "recipients": [
How to validate if array items are string only (it should reject handling if object is in the array item position)?
'class-validator' injected successfully, and it produces some validation results for my API.
You need to tell class-validator to run the validations on each item of the array.  Change your payload DTO to the following:
import { ArrayMinSize, IsArray, IsString } from 'class-validator';
export class ReminderPayloadDto {
  // ...
  @IsArray()
  // "each" tells class-validator to run the validation on each item of the array
  @IsString({ each: true })
  @ArrayMinSize(1)
  recipients: string[];
Link to the docs on this.
                Brilliant! You've missed @IsNotEmpty({ each: true }), but I've got the idea and missed documentation part
– Sergii
                Oct 4, 2021 at 15:33
                @Mahmoud, @IsString() verifies only input value type. It fails if input value is number type for example.
– Sergii
                Sep 23, 2023 at 18:50
For someone who wants to validate specific strings in array:
class MyDto {
    @IsIn(['monday', 'tuesday', 'wednesday', 'thursday', 'friday'], { each: true })
    weekdays: string[];
    // regex mask validation
    @Matches('^[a-zA-Z\\s]+$', undefined, { each: true })
    words: string[];
    @Contains('hello', { each: true })
    greetings: string[];
For custom validation:
import {
  ArrayNotEmpty,
  IsArray,
  Validate,
  ValidateNested,
  ValidatorConstraint,
  ValidatorConstraintInterface
} from 'class-validator'
@ValidatorConstraint({ name: 'arrayPrefixValidator' })
export class ArrayPrefixValidator implements ValidatorConstraintInterface {
  validate(values: string[] = []): boolean {
    if (values.length) {
      return values.every((value) => value.startsWith('user-'))
    return false
class MyDto {
    // Each item contains a prefix str-
    @Validate(ArrayPrefixValidator, { message: 'No user- prefix' })
    accounts: string[];
For more information go to official docs
        
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