Converts JS objects to JSON Schema
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README.md

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to-json-schema

Converts javascript objects (and other types) to corresponding JSON schema

Install

npm install to-json-schema

Example usage

const toJsonSchema = require('to-json-schema');

const objToBeConverted = {
  name: 'David',
  rank: 7,
  born: '1990-04-05T15:09:56.704Z',
  luckyNumbers: [7, 77, 5]
};

const schema = toJsonSchema(objToBeConverted);

schema generated from above code will look like this:

{
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    },
    "rank": {
      "type": "integer"
    },
    "born": {
      "type": "string",
      "format": "date-time"
    },
    "luckyNumbers": {
      "type": "array",
      "items": {
        "type": "integer"
      }
    }
  }
}

toJsonSchema(value, options)

to-json-schema exports function that converts most javascript values to JSON schema. Such a schema can be used to further validation of similar objects/values

  • value: required Any javascript value
  • options: optional options object

Options

Common options

Possible option values

required (true|false default is false) specify true to make all properties required.

const schema = toJsonSchema(33, {required: false});
/*
{
  "type": "integer"
}
*/
const schema = toJsonSchema(33, {required: true});
/*
{
  "type": "integer",
  "required": true
}
*/

postProcessFnc (function)

parameters:

  • type (string) - JSON schema type of the value
  • schema (object) - Generated JSON schema
  • value - (any) - input value
  • defaultFunc (function) - standard function that is used to post-process generated schema. Takes the type, schema, value params.

By providing postProcessFnc, you can modify or replace generated schema. This function will be called recursively for all the properties and sub-properties and array items from leaves to the root. If you want to preserve default functionality, don't forget to call defaultFunc which is currently responsible for setting required for the schema items if there is common option required set tu true.

Following example is showing configuration options leading to all integer values to be automatically required

const options = {
  postProcessFnc: (type, schema, value, defaultFunc) =>
    (type === 'integer') ? {...schema, required: true} : defaultFunc(type, schema, value),
}

const instance = {
  a: 1,
  b: 'str',
}

const schema = toJsonSchema(instance, options);
/*
{
  type: 'object',
  properties: {
    a: {type: 'integer', required: true},
    b: {type: 'string'},
  },
}*/

Arrays options

arrays.mode (all|first|uniform default is all)

all option causes parser to go through all array items, finding the most compatible yet most descriptive schema possible.

Array items are all of compatible type:

const arr = [33, 44, 55];
const schema = toJsonSchema(arr, {arrays: {mode: 'all'}});
/*
{
  "type": "array",
  "items": {
    "type": "integer"
  }
}
*/

Items' types are incompatible. Type is omitted in schema to be able to validate input object:

const arr = [33, 'str', 55];
const schema = toJsonSchema(arr, {arrays: {mode: 'all'}});
/* 
{
  "type": "array"
}
*/

Incompatible in sub-item. Schema still describes object properties

const arr = [
  {name: 'john', grades: [1, 2, 3]},
  {name: 'david', grades: ['a', 'b', 'c']}
];
const schema = toJsonSchema(arr, {arrays: {mode: 'all'}});
/*
{
  "type": "array",
  "items": {
    "type": "object",
    "properties": {
      "name": {
        "type": "string"
      },
      "grades": {
        "type": "array" // due to incompatible array items' types, `items` field is omitted
      }
    }
  }
}
*/

first option takes only first item in the array into account. If performance is a concern, you may consider this option.

const arr = ['str', 11, 30];
const schema = toJsonSchema(arr, {arrays: {mode: 'first'}});
/* Other than first array item is ignored
{
  "type": "array",
  "items": {
    "type": "string"
  }
}
*/

uniform option requires all items in array to have same structure (to convert to the same schema). If not, error is thrown.

const arr = ['str', 11, 30];
const schema = toJsonSchema(arr, {arrays: {mode: 'uniform'}});
/*
 Above code will throw 'Error: Invalid schema, incompatible array items'
*/

Objects options

objects.additionalProperties (boolean, default true)

if set to false, all object schemas will include JSON schema property additionalProperties: false which makes generated schema to perevent any extra properties.

const options = {
  objects: {additionalProperties: false},
}
const obj = {
  a: {
    c: 1,
    d: 1,
  },
  b: 'str',
}
const schema = toJsonSchema(obj, options);
/*
{
  type: 'object',
  properties: {
    a: {
      type: 'object',
      properties: {
        c: {type: 'integer'},
        d: {type: 'integer'},
      },
      additionalProperties: false,
    },
    b: {type: 'string'},
  },
  additionalProperties: false,
}
*/

objects.preProcessFnc (function)

parameters:

  • obj - (object) - input object value that is supposed to be converted into JSON schema
  • defaultFunc (function) - standard function that is used to generate schema from object. Takes just the obj param.

By providing custom function you will be able to modify any object value (including nested ones) and pre-process it before it gets converted into schema or modify generated schema or do the schema conversion entirely by yourself.

Custom function from example bellow ignores all properties other than a and b from input object:

const options = {
  objects: {
    preProcessFnc: (obj, defaultFnc) => defaultFnc({a: obj.a, b: obj.b})
  }
};
const obj = {a: 1, b: 2, c: 3};
const schema = toJsonSchema(obj, options);
/*
{
  "type": "object",
  "properties": {
    "a": {
      "type": "integer"
    },
    "b": {
      "type": "integer",
    }
  }
}
*/

objects.postProcessFnc (function)

parameters:

  • schema (object) - Generated JSON schema
  • obj - (object) - input value
  • defaultFunc (function) - standard function that is used to post-process generated schema. Takes the schema, obj params.

By providing postProcessFnc, you can modify or replace generated schema. This function will be called recursively for all the properties and sub-properties and array items from leaves to the root of the obj object.

Custom objects.postProcessFnc makes properties required on parent type level:

const options = {
  objects: {
    postProcessFnc: (schema, obj, defaultFnc) => ({...defaultFnc(schema, obj), required: Object.getOwnPropertyNames(obj)})
  }
};
const obj = {a: 1, b: 'str'};
const schema = toJsonSchema(obj, options);
/*
{
  type: 'object',
  properties: {
    a: {type: 'integer'},
    b: {type: 'string'},
  }
  required: ['a', 'b']
}
*/

strings options

strings.preProcessFnc (function)

By providing custom function you will be able to modify any string value (including nested ones) and pre-process it before it gets converted to schema, modify generated schema or do the schema conversion entirely by yourself.

Provided function will receive two parameters:

  • string to be converted into JSON schema
  • default function that normally generates the schema. This function receives only string to be converted to JSON schema

Custom function from example bellow converts any string of object containing string to JSON schema and if string's content is 'date' than sets the format property to 'date':

const options = {
  strings: {
    preProcessFnc: (value, defaultFnc) => {
      const schema = defaultFnc(value);
      if (value === 'date') {
        schema.format = 'date';
      }
      return schema;
    },
  },
}
const schema = toJsonSchema('date', options);
/*
{
  "type": "string",
  "format": "date"
}
*/

strings.detectFormat (true|false default id true)

When set to true format of the strings values may be detected based on it's content.

These JSON schema string formats can be detected:

  • date-time
  • date
  • time
  • utc-millisec
  • color
  • style
  • phone
  • uri
  • email
  • ip-address
  • ipv6
const obj = {
  a: '2012-07-08T16:41:41.532Z',
  b: '+31 42 123 4567',
  c: 'http://www.google.com/',
  d: 'obama@whitehouse.gov'
};
const schema = toJsonSchema(obj, {strings: {detectFormat: true}});
/*
{
  "type": "object",
  "properties": {
    "a": {
      "type": "string",
      "format": "date-time"
    },
    "b": {
      "type": "string",
      "format": "phone"
    },
    "c": {
      "type": "string",
      "format": "uri"
    },
    "d": {
      "type": "string",
      "format": "email"
    }
  }
}
*/