To help you get started, we've collected the most common ways that fast-json-stringify is being used within popular public projects.

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.

How it works

fast-json-stringify requires a JSON Schema Draft 7 input to generate a fast stringify function.

Benchmarks

• Machine: EX41S-SSD, Intel Core i7, 4Ghz, 64GB RAM, 4C/8T, SSD.
• Node.js v18.12.1

FJS creation x 4,129 ops/sec ±0.82% (92 runs sampled)
CJS creation x 184,196 ops/sec ±0.12% (97 runs sampled)
AJV Serialize creation x 61,130,591 ops/sec ±0.40% (92 runs sampled)
JSON.stringify array x 5,057 ops/sec ±0.10% (100 runs sampled)
fast-json-stringify array default x 6,243 ops/sec ±0.14% (98 runs sampled)
fast-json-stringify array json-stringify x 6,261 ops/sec ±0.30% (99 runs sampled)
compile-json-stringify array x 6,842 ops/sec ±0.18% (96 runs sampled)
AJV Serialize array x 6,964 ops/sec ±0.11% (95 runs sampled)
JSON.stringify large array x 248 ops/sec ±0.07% (90 runs sampled)
fast-json-stringify large array default x 99.96 ops/sec ±0.22% (74 runs sampled)
fast-json-stringify large array json-stringify x 248 ops/sec ±0.07% (90 runs sampled)
compile-json-stringify large array x 317 ops/sec ±0.09% (89 runs sampled)
AJV Serialize large array x 111 ops/sec ±0.07% (33 runs sampled)
JSON.stringify long string x 16,002 ops/sec ±0.09% (98 runs sampled)
fast-json-stringify long string x 15,979 ops/sec ±0.09% (96 runs sampled)
compile-json-stringify long string x 15,952 ops/sec ±0.31% (97 runs sampled)
AJV Serialize long string x 21,416 ops/sec ±0.08% (98 runs sampled)
JSON.stringify short string x 12,944,272 ops/sec ±0.09% (96 runs sampled)
fast-json-stringify short string x 30,585,790 ops/sec ±0.27% (97 runs sampled)
compile-json-stringify short string x 30,656,406 ops/sec ±0.12% (96 runs sampled)
AJV Serialize short string x 30,406,785 ops/sec ±0.37% (96 runs sampled)
JSON.stringify obj x 3,153,043 ops/sec ±0.33% (99 runs sampled)
fast-json-stringify obj x 6,866,434 ops/sec ±0.11% (100 runs sampled)
compile-json-stringify obj x 15,886,723 ops/sec ±0.15% (98 runs sampled)
AJV Serialize obj x 8,969,043 ops/sec ±0.36% (97 runs sampled)
JSON stringify date x 1,126,547 ops/sec ±0.09% (97 runs sampled)
fast-json-stringify date format x 1,836,188 ops/sec ±0.12% (99 runs sampled)
compile-json-stringify date format x 1,125,735 ops/sec ±0.19% (98 runs sampled)

Table of contents:

• Example
• Options
• API
• fastJsonStringify
• Specific use cases
• Required
• Missing fields
• Pattern Properties
• Additional Properties
• AnyOf and OneOf
• Reuse - $ref
• Long integers
• Integers
• Nullable
• Large Arrays
• Security Notice
• Debug Mode
• Standalone Mode
• Acknowledgements
• License

Try it out on RunKit: https://runkit.com/npm/fast-json-stringify

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 v8 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
• largeArrayMechanism: set the mechanism that should be used to handle large (by default 20000 or more items) arrays. More details

API

fastJsonStringify(schema)

Build a stringify() function based on jsonschema draft 7 spec.

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 Date object:

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

const date = new Date()
console.log(stringify(date)) // '"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: 'mail@example.com'
}

console.log(stringify(obj)) // '{"mail":"mail@example.com"}'

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 serialized 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 and OneOf

fast-json-stringify supports the anyOf and oneOf keywords as defined by JSON schema. Both 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 and oneOf use 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 handle automatically BigInt.

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, floor or trunc. Default is trunc:

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
• object -> {}
• array -> []

Large Arrays

Large arrays are, for the scope of this document, defined as arrays containing, by default, 20000 elements or more. That value can be adjusted via the option parameter largeArraySize.

At some point the overhead caused by the default mechanism used by fast-json-stringify to handle arrays starts increasing exponentially, leading to slow overall executions.

Settings

In order to improve that the user can set the largeArrayMechanism and largeArraySize options.

largeArrayMechanism's default value is default. Valid values for it are:

• default - This option is a compromise between performance and feature set by still providing the expected functionality out of this lib but giving up some possible performance gain. With this option set, large arrays would be stringified by joining their stringified elements using Array.join instead of string concatenation for better performance
• json-stringify - This option will remove support for schema validation within large arrays completely. By doing so the overhead previously mentioned is nulled, greatly improving execution time. Mind there's no change in behavior for arrays not considered large

largeArraySize's default value is 20000. Valid values for it are integer-like values, such as:

• 20000
• 2e4
• '20000'
• '2e4' - note this will be converted to 2, not 20000
• 1.5 - note this will be converted to 1

Unsafe string

By default, the library escapes all strings. With the 'unsafe' format, the string isn't escaped. This has a potentially dangerous security issue. You can use it only if you are sure that your data doesn't need escaping. The advantage is a significant performance improvement.

Example:

const stringify = fastJson({
  title: 'Example Schema',
  type: 'object',
  properties: {
    'code': {
	    type: 'string',
	    format 'unsafe'
    }
  }
})

Benchmarks

For reference, here goes some benchmarks for comparison over the three mechanisms. Benchmarks conducted on an old machine.

• Machine: ST1000LM024 HN-M 1TB HDD, Intel Core i7-3610QM @ 2.3GHz, 12GB RAM, 4C/8T.
• Node.js v16.13.1

JSON.stringify large array x 157 ops/sec ±0.73% (86 runs sampled)
fast-json-stringify large array default x 48.72 ops/sec ±4.92% (48 runs sampled)
fast-json-stringify large array json-stringify x 157 ops/sec ±0.76% (86 runs sampled)
compile-json-stringify large array x 175 ops/sec ±4.47% (79 runs sampled)
AJV Serialize large array x 58.76 ops/sec ±4.59% (60 runs sampled)

Security notice

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

To achieve low cost and high 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.

Users are responsible for sending trusted data. fast-json-stringify guarantees that you will get a valid output only if your input matches the schema or can be coerced to the schema. If your input doesn't match the schema, you will get undefined behavior.

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'
    }
  }
}, { mode: 'debug' })

console.log(debugCompiled) // it is a object contain code, ajv instance
const rawString = debugCompiled.code // it is the generated code
console.log(rawString) 

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

Standalone Mode

The standalone mode is used to compile the code that can be directly run by node itself. You need to have fast-json-stringify installed for the standalone code to work.

const fs = require('fs')
const code = fastJson({
  title: 'default string',
  type: 'object',
  properties: {
    firstName: {
      type: 'string'
    }
  }
}, { mode: 'standalone' })

fs.writeFileSync('stringify.js', code)
const stringify = require('stringify.js')
console.log(stringify({ firstName: 'Foo', surname: 'bar' })) // '{"firstName":"Foo"}'

Acknowledgements

This project was kindly sponsored by nearForm.

License

MIT