@alexkuz/json-schema-default-instance NPM

json-schema-default-instance

Creates an object as an instance of the given schema using its default properties.

Accepts multiple schemas, referenced by id.
Resolves $ref and allOf.
Instantiates all properties that have a default.
If requiredOnly = true, ignores properties that are not listed in the required array (see example below).
If resolveDefaultRefs = true, also resolves references within defaults.

Usage

Install with npm install --save json-schema-default-instance

See test/test.js for an example with $ref and allOf.

Simple example below:

const {Instantiator} = require('json-schema-default-instance');
const mySchemas = [
  {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "id": "theSchemaId",
    "type": "object",
    "required": [
      "firstName",
      "lastName"
    ],
    "properties": {
      "firstName": {
        "type": "string",
        "default": "Foo"
      },
      "lastName": {
        "type": "string",
        "default": "Bar"
      },
      "optionalProperty": {
        "type": "string",
        "default": "Hello"
      }
    }
  }
]

/* Instantiate with all properties */
let ins = new Instantiator(mySchemas);
let myDefaultInstance = ins.instantiate('theSchemaId');
console.log(myDefaultInstance);

// => { firstName: 'Foo', lastName: 'Bar', optionalProperty: 'Hello' }

/* This time, only with `required` properties */
ins.requiredOnly = true;
let myRequiredInstance = ins.instantiate('theSchemaId');
console.log(myRequiredInstance);

// => { firstName: 'Foo', lastName: 'Bar' }

Notes

Relies heavily on Ajv for caching and lookup by ref, even though no validation is done here. Ajv: Another JSON Schema Validator

Ajv does have its own useDefaults option which can be used instead of this package, but it does not support default keywords in subschemas or allOf. If you don't need allOf, just use Ajv directly (see Ajv assigning-defaults and related discussion).

Performance: this code recursively walks through the schema on each call, which can negatively impact performance. Depending on the use case, it may make sense to precompute the defaults and cache them to an object or module.

Need help understanding JSON Schema? I would recommend the Space Telescope Science Institute's Understanding JSON Schema