fast-json-stringify

fast-json-stringify is significantly faster than JSON.stringify() for small payloads. Its performance advantage shrinks as your payload grows. It pairs well with flatstr, which triggers a V8 optimization that improves performance when eventually converting the string to a Buffer.

Benchmarks

Machine: EX41S-SSD, Intel Core i7, 4Ghz, 64GB RAM, 4C/8T, SSD.
Node.js v16.9.1

FJS creation x 6,040 ops/sec ±1.17% (91 runs sampled)

JSON.stringify array x 5,519 ops/sec ±0.08% (99 runs sampled)
fast-json-stringify array x 7,143 ops/sec ±0.14% (97 runs sampled)

JSON.stringify long string x 16,438 ops/sec ±0.32% (98 runs sampled)
fast-json-stringify long string x 16,457 ops/sec ±0.09% (97 runs sampled)

JSON.stringify short string x 12,061,258 ops/sec ±0.32% (97 runs sampled)
fast-json-stringify short string x 35,531,071 ops/sec ±0.17% (94 runs sampled)

JSON.stringify obj x 3,079,746 ops/sec ±0.09% (95 runs sampled)
fast-json-stringify obj x 7,721,569 ops/sec ±0.12% (98 runs sampled)

JSON stringify date x 1,149,786 ops/sec ±0.10% (99 runs sampled)
fast-json-stringify date format x 1,674,498 ops/sec ±0.12% (99 runs sampled)

Example

const fastJson = require('fast-json-stringify')
const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    firstName: {
      type: 'string'
    },
    lastName: {
      type: 'string'
    },
    age: {
      description: 'Age in years',
      type: 'integer'
    },
    reg: {
      type: 'string'
    }
  }
})

console.log(stringify({
  firstName: 'Matteo',
  lastName: 'Collina',
  age: 32,
  reg: /"([^"]|\\")*"/
}))

Options

Optionally, you may provide to fast-json-stringify an option object as second parameter:

const fastJson = require('fast-json-stringify')
const stringify = fastJson(mySchema, {
  schema: { ... },
  ajv: { ... },
  rounding: 'ceil'
})

schema: external schemas references by $ref property. More details
ajv: ajv instance's settings for those properties that require ajv. More details
rounding: setup how the integer types will be rounded when not integers. More details

API

fastJsonStringify(schema)

Build a stringify() function based on jsonschema.

Supported types:

'string'
'integer'
'number'
'array'
'object'
'boolean'
'null'

And nested ones, too.

Specific use cases

Instance	Serialized as
`Date`	`string` via `toISOString()`
`RegExp`	`string`
`BigInt`	`integer` via `toString`

JSON Schema built-in formats for dates are supported and will be serialized as:

Format	Serialized format example
`date-time`	`2020-04-03T09:11:08.615Z`
`date`	`2020-04-03`
`time`	`09:11:08`

Note: In the case of string formatted Date and not Date Object, there will be no manipulation on it. It should be properly formatted.

Example with a MomentJS object:

const moment = require('moment')

const stringify = fastJson({
  title: 'Example Schema with string date-time field',
  type: 'string',
  format: 'date-time'
})

console.log(stringify(moment())) // '"YYYY-MM-DDTHH:mm:ss.sssZ"'

Required

You can set specific fields of an object as required in your schema by adding the field name inside the required array in your schema. Example:

const schema = {
  title: 'Example Schema with required field',
  type: 'object',
  properties: {
    nickname: {
      type: 'string'
    },
    mail: {
      type: 'string'
    }
  },
  required: ['mail']
}

If the object to stringify is missing the required field(s), fast-json-stringify will throw an error.

Missing fields

If a field is present in the schema (and is not required) but it is not present in the object to stringify, fast-json-stringify will not write it in the final string. Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      type: 'string'
    },
    mail: {
      type: 'string'
    }
  }
})

const obj = {
  mail: '[email protected]'
}

console.log(stringify(obj)) // '{"mail":"[email protected]"}'

Defaults

fast-json-stringify supports default jsonschema key in order to serialize a value if it is undefined or not present.

Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      type: 'string',
      default: 'the default string'
    }
  }
})

console.log(stringify({})) // '{"nickname":"the default string"}'
console.log(stringify({nickname: 'my-nickname'})) // '{"nickname":"my-nickname"}'

Pattern properties

fast-json-stringify supports pattern properties as defined by JSON schema. patternProperties must be an object, where the key is a valid regex and the value is an object, declared in this way: { type: 'type' }. patternProperties will work only for the properties that are not explicitly listed in the properties object. Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      type: 'string'
    }
  },
  patternProperties: {
    'num': {
      type: 'number'
    },
    '.*foo$': {
      type: 'string'
    }
  }
})

const obj = {
  nickname: 'nick',
  matchfoo: 42,
  otherfoo: 'str'
  matchnum: 3
}

console.log(stringify(obj)) // '{"matchfoo":"42","otherfoo":"str","matchnum":3,"nickname":"nick"}'

Additional properties

fast-json-stringify supports additional properties as defined by JSON schema. additionalProperties must be an object or a boolean, declared in this way: { type: 'type' }. additionalProperties will work only for the properties that are not explicitly listed in the properties and patternProperties objects.

If additionalProperties is not present or is set to false, every property that is not explicitly listed in the properties and patternProperties objects,will be ignored, as described in Missing fields. Missing fields are ignored to avoid having to rewrite objects before serializing. However, other schema rules would throw in similar situations. If additionalProperties is set to true, it will be used by JSON.stringify to stringify the additional properties. If you want to achieve maximum performance, we strongly encourage you to use a fixed schema where possible. The additional properties will always be serialzied at the end of the object. Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      type: 'string'
    }
  },
  patternProperties: {
    'num': {
      type: 'number'
    },
    '.*foo$': {
      type: 'string'
    }
  },
  additionalProperties: {
    type: 'string'
  }
})

const obj = {
  nickname: 'nick',
  matchfoo: 42,
  otherfoo: 'str'
  matchnum: 3,
  nomatchstr: 'valar morghulis',
  nomatchint: 313
}

console.log(stringify(obj)) // '{"nickname":"nick","matchfoo":"42","otherfoo":"str","matchnum":3,"nomatchstr":"valar morghulis",nomatchint:"313"}'

AnyOf

fast-json-stringify supports the anyOf keyword as defined by JSON schema. anyOf must be an array of valid JSON schemas. The different schemas will be tested in the specified order. The more schemas stringify has to try before finding a match, the slower it will be.

anyOf uses ajv as a JSON schema validator to find the schema that matches the data. This has an impact on performance—only use it as a last resort.

Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    'undecidedType': {
      'anyOf': [{
	type: 'string'
      }, {
	type: 'boolean'
      }]
    }
  }
})

When specifying object JSON schemas for anyOf, add required validation keyword to match only the objects with the properties you want.

Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'array',
  items: {
    anyOf: [
      {
        type: 'object',
        properties: {
          savedId: { type: 'string' }
        },
        // without "required" validation any object will match
        required: ['savedId']
      },
      {
        type: 'object',
        properties: {
          error: { type: 'string' }
        },
        required: ['error']
      }
    ]
  }
})

If/then/else

fast-json-stringify supports if/then/else jsonschema feature. See ajv documentation.

Example:

const stringify = fastJson({
  'type': 'object',
  'properties': {
  },
  'if': {
    'properties': {
      'kind': { 'type': 'string', 'enum': ['foobar'] }
    }
  },
  'then': {
    'properties': {
      'kind': { 'type': 'string', 'enum': ['foobar'] },
      'foo': { 'type': 'string' },
      'bar': { 'type': 'number' }
    }
  },
  'else': {
    'properties': {
      'kind': { 'type': 'string', 'enum': ['greeting'] },
      'hi': { 'type': 'string' },
      'hello': { 'type': 'number' }
    }
  }
})

console.log(stringify({
  kind: 'greeting',
  foo: 'FOO',
  bar: 42,
  hi: 'HI',
  hello: 45
})) // {"kind":"greeting","hi":"HI","hello":45}
console.log(stringify({
  kind: 'foobar',
  foo: 'FOO',
  bar: 42,
  hi: 'HI',
  hello: 45
})) // {"kind":"foobar","foo":"FOO","bar":42}

NB Do not declare the properties twice or you will print them twice!

Reuse - $ref

If you want to reuse a definition of a value, you can use the property $ref. The value of $ref must be a string in JSON Pointer format. Example:

const schema = {
  title: 'Example Schema',
  definitions: {
    num: {
      type: 'object',
      properties: {
        int: {
          type: 'integer'
        }
      }
    },
    str: {
      type: 'string'
    }
  },
  type: 'object',
  properties: {
    nickname: {
      $ref: '#/definitions/str'
    }
  },
  patternProperties: {
    'num': {
      $ref: '#/definitions/num'
    }
  },
  additionalProperties: {
    $ref: '#/definitions/def'
  }
}

const stringify = fastJson(schema)

If you need to use an external definition, you can pass it as an option to fast-json-stringify. Example:

const schema = {
  title: 'Example Schema',
  type: 'object',
  properties: {
    nickname: {
      $ref: 'strings#/definitions/str'
    }
  },
  patternProperties: {
    'num': {
      $ref: 'numbers#/definitions/num'
    }
  },
  additionalProperties: {
    $ref: 'strings#/definitions/def'
  }
}

const externalSchema = {
  numbers: {
    definitions: {
      num: {
        type: 'object',
        properties: {
          int: {
            type: 'integer'
          }
        }
      }
    }
  },
  strings: require('./string-def.json')
}

const stringify = fastJson(schema, { schema: externalSchema })

External definitions can also reference each other. Example:

const schema = {
  title: 'Example Schema',
  type: 'object',
  properties: {
    foo: {
      $ref: 'strings#/definitions/foo'
    }
  }
}

const externalSchema = {
  strings: {
    definitions: {
      foo: {
        $ref: 'things#/definitions/foo'
      }
    }
  },
  things: {
    definitions: {
      foo: {
        type: 'string'
      }
    }
  }
}

const stringify = fastJson(schema, { schema: externalSchema })

Long integers

By default the library will automatically handle BigInt from Node.js v10.3 and above. If you can't use BigInts in your environment, long integers (64-bit) are also supported using the long module. Example:

// => using native BigInt
const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    id: {
      type: 'integer'
    }
  }
})

const obj = {
  id: 18446744073709551615n
}

console.log(stringify(obj)) // '{"id":18446744073709551615}'

// => using the long library
const Long = require('long')

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    id: {
      type: 'integer'
    }
  }
})

const obj = {
  id: Long.fromString('18446744073709551615', true)
}

console.log(stringify(obj)) // '{"id":18446744073709551615}'

Integers

The type: integer property will be truncated if a floating point is provided. You can customize this behaviour with the rounding option that will accept round, ceil or floor:

const stringify = fastJson(schema, { rounding: 'ceil' })

Nullable

According to the Open API 3.0 specification, a value that can be null must be declared nullable.

Nullable object

const stringify = fastJson({
  'title': 'Nullable schema',
  'type': 'object',
  'nullable': true,
  'properties': {
    'product': {
      'nullable': true,
      'type': 'object',
      'properties': {
        'name': {
          'type': 'string'
        }
      }
    }
  }
})

console.log(stringify({product: {name: "hello"}})) // "{"product":{"name":"hello"}}"
console.log(stringify({product: null})) // "{"product":null}"
console.log(stringify(null)) // null

Otherwise, instead of raising an error, null values will be coerced as follows:

integer -> 0
number -> 0
string -> ""
boolean -> false

Security notice

Treat the schema definition as application code, it is not safe to use user-provided schemas.

In order to achieve lowest cost/highest performance redaction fast-json-stringify creates and compiles a function (using the Function constructor) on initialization. While the schema is currently validated for any developer errors, there is no guarantee that supplying user-generated schema could not expose your application to remote attacks.

Debug Mode

The debug mode can be activated during your development to understand what is going on when things do not work as you expect.

const debugCompiled = fastJson({
  title: 'default string',
  type: 'object',
  properties: {
    firstName: {
      type: 'string'
    }
  }
}, { debugMode: true })

console.log(debugCompiled) // it is an array of functions that can create your `stringify` function
console.log(debugCompiled.toString()) // print a "ready to read" string function, you can save it to a file

const rawString = debugCompiled.toString()
const stringify = fastJson.restore(rawString) // use the generated string to get back the `stringify` function
console.log(stringify({ firstName: 'Foo', surname: 'bar' })) // '{"firstName":"Foo"}'

Acknowledgements

This project was kindly sponsored by nearForm.

License

MIT

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Benchmarks

Table of contents:

Example

Options

API

fastJsonStringify(schema)

Specific use cases

Required

Missing fields

Defaults

Pattern properties

Additional properties

AnyOf

If/then/else

Reuse - $ref

Long integers

Integers

Nullable

Nullable object

Security notice

Debug Mode

Acknowledgements

License