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At the REST layer, request body is being validated against the OpenAPI schema specification.

Type Validation

Type validation comes out-of-the-box in LoopBack.

Validation is applied on the parameters and the request body data. It also uses OpenAPI specification as the reference to infer the validation rules.

Take the capacity property in the CoffeeShop model as an example; it is a number. When creating a CoffeeShop by calling /POST, if a string is specified for the capacity property as below:

{
  "city": "Toronto",
  "phoneNum": "416-111-1111",
  "capacity": "100"
}

a “request body is invalid” error is expected:

{
  "error": {
    "statusCode": 422,
    "name": "UnprocessableEntityError",
    "message": "The request body is invalid. See error object `details` property for more info.",
    "code": "VALIDATION_FAILED",
    "details": [
      {
        "path": ".capacity",
        "code": "type",
        "message": "should be number",
        "info": {
          "type": "number"
        }
      }
    ]
  }
}

Validation against OpenAPI Schema Specification

For validation against an OpenAPI schema specification, the AJV module is used to validate data with a JSON schema generated from the OpenAPI schema specification. More details can be found about validation keywords and annotation keywords available in AJV. AJV can also be extended with custom keywords and formats, see AJV defining custom keywords page.

Besides AJV, other third-party validation libraries, such as @hapi/joi and class-validator, can be used.

Below are a few examples of using AJV for validation. The source code of the snippets can be found in the coffee-shop.model.ts in the example app.

Example 1: Length limit

A typical validation example is to have a length limit on a string using the keywords maxLength and minLength. For example:

/src/models/coffee-shop.model.ts

  @property({
    type: 'string',
    required: true,
    // --- add jsonSchema -----
    jsonSchema: {
      maxLength: 10,
      minLength: 1,
    },
    // ------------------------
  })
  city: string;

If the city property in the request body does not satisfy the requirement as follows:

{
  "city": "a long city name 123123123",
  "phoneNum": "416-111-1111",
  "capacity": 10
}

an error will occur with details on what has been violated:

{
  "error": {
    "statusCode": 422,
    "name": "UnprocessableEntityError",
    "message": "The request body is invalid. See error object `details` property for more info.",
    "code": "VALIDATION_FAILED",
    "details": [
      {
        "path": ".city",
        "code": "maxLength",
        "message": "should NOT be longer than 10 characters",
        "info": {
          "limit": 10
        }
      }
    ]
  }
}

Example 2: Value range for a number

For numbers, the validation rules are used to specify the range of the value. For example, any coffee shop would not be able to have more than 100 people, it can be specified as follows:

/src/models/coffee-shop.model.ts

  @property({
    type: 'number',
    required: true,
    // --- add jsonSchema -----
    jsonSchema: {
      maximum: 100,
      minimum: 1,
    },
    // ------------------------
  })
  capacity: number;

Example 3: Pattern in a string

Model properties, such as phone number and postal/zip code, usually have certain patterns. In this case, the pattern keyword is used to specify the restrictions.

Below shows an example of the expected pattern of phone numbers, i.e. a sequence of 10 digits separated by - after the 3rd and 6th digits.

/src/models/coffee-shop.model.ts

  @property({
    type: 'string',
    required: true,
    // --- add jsonSchema -----
    jsonSchema: {
      pattern: '\\d{3}-\\d{3}-\\d{4}',
    },
    // ------------------------
  })
  phoneNum: string;

Customize validation errors

Since the error is being caught at the REST layer, the simplest way to customize the errors is to customize the sequnce. It exists in all LoopBack applications scaffolded by using the lb4 command and can be found in src/sequence.ts.

Let’s take a closer look at how to customize the error. A few things to note in the below code snippet:

  1. inject RestBindings.SequenceActions.LOG_ERROR for logging error and RestBindings.ERROR_WRITER_OPTIONS for options
  2. customize error for particular endpoints
  3. create a new error with customized properties
  4. log the error using RestBindings.SequenceActions.LOG_ERROR

/src/sequence.ts

export class MySequence implements SequenceHandler {
  // 1. inject RestBindings.SequenceActions.LOG_ERROR for logging error
  // and RestBindings.ERROR_WRITER_OPTIONS for options
  constructor(
    /*..*/
    @inject(RestBindings.SequenceActions.LOG_ERROR)
    protected logError: LogError,
    @inject(RestBindings.ERROR_WRITER_OPTIONS, {optional: true})
    protected errorWriterOptions?: ErrorWriterOptions,
  ) {}

  async handle(context: RequestContext) {
    try {
      // ...
    } catch (err) {
      this.handleError(context, err as HttpErrors.HttpError);
    }
  }

  /**
   * Handle errors
   * If the request url is `/coffee-shops`, customize the error message.
   */
  handleError(context: RequestContext, err: HttpErrors.HttpError) {
    // 2. customize error for particular endpoint
    if (context.request.url === '/coffee-shops') {
      // if this is a validation error
      if (err.statusCode === 422) {
        const customizedMessage = 'My customized validation error message';

        let customizedProps = {};
        if (this.errorWriterOptions?.debug) {
          customizedProps = {stack: err.stack};
        }

        // 3. Create a new error with customized properties
        // you can change the status code here too
        const errorData = {
          statusCode: 422,
          message: customizedMessage,
          resolution: 'Contact your admin for troubleshooting.',
          code: 'VALIDATION_FAILED',
          ...customizedProps,
        };

        context.response.status(422).send(errorData);

        // 4. log the error using RestBindings.SequenceActions.LOG_ERROR
        this.logError(err, err.statusCode, context.request);

        // The error was handled
        return;
      }
    }

    // Otherwise fall back to the default error handler
    this.reject(context, err);
  }
}