JSON Schema containing array with item reference gives Typescript error: Type '"object"' is not assignable to type '"array" | undefined'.

What version of Ajv are you using? Does the issue happen if you use the latest version? Libraries declared in package.json:

  "dependencies": {
    "ajv": "^8.6.0"
  },
  "devDependencies": {
    "@types/node": "^15.12.2",
    "@typescript-eslint/eslint-plugin": "^4.27.0",
    "@typescript-eslint/parser": "^4.27.0",
    "eslint": "^7.28.0",
    "eslint-config-prettier": "^8.3.0",
    "eslint-plugin-node": "^11.1.0",
    "eslint-plugin-prettier": "^3.4.0",
    "prettier": "^2.3.1",
    "ts-node": "^10.0.0",
    "typescript": "^4.3.2"
  }

Your typescript code Based on the Recursion example here: https://json-schema.org/understanding-json-schema/structuring.html

/* eslint-disable no-console */
// eslint-disable-next-line node/no-missing-import
import Ajv, { JSONSchemaType } from 'ajv/dist/2020';
import draft7MetaSchema from 'ajv/dist/refs/json-schema-draft-07.json';
const ajv = new Ajv();
ajv.addMetaSchema(draft7MetaSchema);

const schema: JSONSchemaType<any> = {
  $schema: 'http://json-schema.org/draft-07/schema#',

  definitions: {
    person: {
      type: 'object',
      properties: {
        name: { type: 'string', nullable: false },
        children: {
          type: 'array',
          minItems: 0,
          items: { $ref: '#/definitions/person' },
          nullable: false
        }
      },
      required: ['name']
    }
  },

  type: 'object',

  properties: {
    person: { $ref: '#/definitions/person' }
  }
};

const validator = ajv.compile(schema);

const validData = {
  person: {
    name: 'Elizabeth',
    children: [
      {
        name: 'Charles',
        children: [
          {
            name: 'William',
            children: [{ name: 'George' }, { name: 'Charlotte' }]
          },
          {
            name: 'Harry'
          }
        ]
      }
    ]
  }
};

if (validator(validData)) {
  console.log(validData);
} else {
  console.log(validator.errors);
}

Typescript compiler error messages

    return new TSError(diagnosticText, diagnosticCodes);
           ^
TSError: ⨯ Unable to compile TypeScript:
src/issue.ts:13:7 - error TS2322: Type '"object"' is not assignable to type '"array" | undefined'.

13       type: 'object',
         ~~~~

    at createTSError (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:587:12)
    at reportTSError (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:591:19)
    at getOutput (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:921:36)
    at Object.compile (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:1189:32)
    at Module.m._compile (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:1295:42)
    at Module._extensions..js (internal/modules/cjs/loader.js:785:10)
    at Object.require.extensions.<computed> [as .ts] (/Users/anjum/Documents/workspace/resilient/playground/json-schema-validator/node_modules/ts-node/src/index.ts:1298:12)
    at Module.load (internal/modules/cjs/loader.js:641:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:837:10)

Describe the change that should be made to address the issue? I do not know. However, I spotted that if I change #/definitions/person/properties/children/nullable to true then the error disappears in VSCode but the Typescript compiler still gives the same error.

Are you going to resolve the issue? No because I do not know why this is happening.

[erikbrinkman]

@asnaseer-resilient One more little comment because I stumbled onto when looking over the tests. They did the smart thing of using:

const schema: JSONSchemaType<MySchema> & {
  definitions: {
    a: JSONSchemaType<...>,
    ...
  }
} = { ... }

to also check the definitions. Note, it won’t make sure that the references to those definitions match the type, just that the individual definitions also type check correctly. Still it will provide a little more guarantees in certain circumstances.

[asnaseer-resilient]

@erikbrinkman Thank you very much for your insights - much appreciated. For now I think I will stick with JSONSchemType as it provides more expressive validations.