jtd: JSON Validation for Dart

JSON Type Definition, aka RFC 8927, is an easy-to-learn, standardized way to define a schema for JSON data. You can use JSON Typedef to portably validate data across programming languages, create dummy data, generate code, and more.

This package is a Dart / Flutter implementation of JSON Type Definition. It lets you validate input data against JSON Type Definition schemas.

Installation

You can install this package with pub

Documentation

For more high-level documentation about JSON Typedef in general see:

• The JSON Typedef Website

Basic Usage

Here's an example of how you can use this package to validate JSON data against a JSON Typedef schema:

import 'package:json_typedef_dart/json_typedef_dart.dart';

Json schema = <String, dynamic>{
   "properties": {
      "name": {"type": "string"},
      "age": {"type": "uint32"},
      "phones": {
         "elements": {"type": "string"}
      }
   }
};

void main() {
// validate returns an array of validation errors. If there were no problems
// with the input, it returns an empty array.

// Outputs: []
   print(validate(schema: schema, data: {
      "name": "John Doe",
              "age": 43,
              "phones": ["+44 1234567", "+44 2345678"],
   }));

// This next input has three problems with it:
//
// 1. It's missing "name", which is a required property.
// 2. "age" is a string, but it should be an integer.
// 3. "phones[1]" is a number, but it should be a string.
//
// Each of those errors corresponds to one of the errors returned by validate.

// Outputs:
//
// [
//   { instancePath: [], schemaPath: [ 'properties', 'name' ] },
//   {
//     instancePath: [ 'age' ],
//     schemaPath: [ 'properties', 'age', 'type' ]
//   },
//   {
//     instancePath: [ 'phones', '1' ],
//     schemaPath: [ 'properties', 'phones', 'elements', 'type' ]
//   }
// ]
   print(validate(schema: schema, data: {
      "age": "43",
              "phones": ["+44 1234567", 442345678],
   }));
}

Advanced Usage: Limiting Errors Returned

By default, validate returns every error it finds. If you just care about whether there are any errors at all, or if you can't show more than some number of errors, then you can get better performance out of validate using the maxErrors option.

For example, taking the same example from before, but limiting it to 1 error, we get:

import 'package:json_typedef_dart/json_typedef_dart.dart';

Json schema = <String, dynamic>{
   "properties": {
      "name": {"type": "string"},
      "age": {"type": "uint32"},
      "phones": {
         "elements": {"type": "string"}
      }
   }
};

void main() {
// Outputs:
//
// [ { instancePath: [], schemaPath: [ 'properties', 'name' ] } ]
   print(validate(schema: schema, data: {
      "age": "43",
              "phones": ["+44 1234567", "+44 2345678"],
   },maxErrors: 1));

}

Advanced Usage: Handling Untrusted Schemas

If you want to run validate against a schema that you don't trust, then you should:

1. Ensure the schema is well-formed, using isValidSchema which validates things like making sure all refs have corresponding definitions.

2. Call validate with the maxDepth option. JSON Typedef lets you write recursive schemas -- if you're evaluating against untrusted schemas, you might go into an infinite loop when evaluating against a malicious input, such as this one:

   {
     "ref": "loop",
     "definitions": {
       "loop": {
         "ref": "loop"
       }
     }
   }

The maxDepth option tells jtd.validate how many refs to follow recursively before giving up and throwing MaxDepthExceededError.

Here's an example of how you can use jtd to evaluate data against an untrusted schema:

import 'package:json_typedef_dart/json_typedef_dart.dart';

// validateUntrusted returns true if `data` satisfies `schema`, and false if it
// does not. Throws an error if `schema` is invalid, or if validation goes in an
// infinite loop.
bool validateUntrusted(Json schema, dynamic data) {
   if (!isValidSchema(schema)) {
      throw Exception("invalid schema");
   }

// You should tune maxDepth to be high enough that most legitimate schemas
// evaluate without errors, but low enough that an attacker cannot cause a
// denial of service attack.
   return validate(schema: schema, data: data, maxDepth: 32).isEmpty;
}

void main() {
// Outputs: true
   print(validateUntrusted(<String, dynamic>{"type": "string"}, "foo"));

// Outputs: false
   validateUntrusted(<String, dynamic>{"type": "string"}, null);

// Throws "invalid schema"
   try {
      validateUntrusted(<String, dynamic>{"type": "nonsense"}, null);
   } catch (e) {
      print(e);
   }
// Throws an instance of MaxDepthExceededError
   try {
      validateUntrusted(<String, dynamic>{
         "ref": "loop",
         "definitions": {
            "loop": {"ref": "loop"}
         }
      }, null);
   }
   catch(e){
      print(e);
   }
}