# fast-json-stringify
[](http://standardjs.com/) [](https://dev.azure.com/fastify/fastify/_build/latest?definitionId=3&branchName=master) [](https://travis-ci.org/fastify/fast-json-stringify) [](https://www.npmjs.com/package/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__](https://www.npmjs.com/package/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 `v10.15.2`
```
FJS creation x 8,951 ops/sec ±0.51% (92 runs sampled)
JSON.stringify array x 5,146 ops/sec ±0.32% (97 runs sampled)
fast-json-stringify array x 8,402 ops/sec ±0.62% (95 runs sampled)
fast-json-stringify-uglified array x 8,474 ops/sec ±0.49% (93 runs sampled)
JSON.stringify long string x 13,061 ops/sec ±0.25% (98 runs sampled)
fast-json-stringify long string x 13,059 ops/sec ±0.21% (98 runs sampled)
fast-json-stringify-uglified long string x 13,099 ops/sec ±0.14% (98 runs sampled)
JSON.stringify short string x 6,295,988 ops/sec ±0.28% (98 runs sampled)
fast-json-stringify short string x 43,335,575 ops/sec ±1.24% (86 runs sampled)
fast-json-stringify-uglified short string x 40,042,871 ops/sec ±1.38% (93 runs sampled)
JSON.stringify obj x 2,557,026 ops/sec ±0.20% (97 runs sampled)
fast-json-stringify obj x 9,001,890 ops/sec ±0.48% (90 runs sampled)
fast-json-stringify-uglified obj x 9,073,607 ops/sec ±0.41% (94 runs sampled)
```
#### Table of contents:
- `Example`
- `API`
- `fastJsonStringify`
- `Specific use cases`
- `Required`
- `Missing fields`
- `Pattern Properties`
- `Additional Properties`
- `AnyOf`
- `Reuse - $ref`
- `Long integers`
- `Uglify`
- `Nullable`
- `Caveat`
- `Acknowledgements`
- `License`
## Example
```js
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: /"([^"]|\\")*"/
}))
```
## API
### fastJsonStringify(schema)
Build a `stringify()` function based on
[jsonschema](http://json-schema.org/).
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` |
#### 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:
```javascript
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:
```javascript
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:
```javascript
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:
```javascript
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.
Example:
```javascript
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)) // '{"matchfoo":"42","otherfoo":"str","matchnum":3,"nomatchstr":"valar morghulis",nomatchint:"313","nickname":"nick"}'
```
#### 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](https://www.npmjs.com/package/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:
```javascript
const stringify = fastJson({
title: 'Example Schema',
type: 'object',
properties: {
'undecidedType': {
'anyOf': [{
type: 'string'
}, {
type: 'boolean'
}]
}
}
}
```
#### If/then/else
`fast-json-stringify` supports `if/then/else` jsonschema feature. See [ajv documentation](https://ajv.js.org/keywords.html#ifthenelse).
Example:
```javascript
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:** don't declare the properties twice or you'll 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](https://tools.ietf.org/html/rfc6901) format.
Example:
```javascript
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:
```javascript
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:
```javascript
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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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](https://github.com/dcodeIO/long.js) module.
Example:
```javascript
// => 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}'
```
#### Uglify
If you want to squeeze a little bit more performance out of the serialization at the cost of readability in the generated code, you can pass `uglify: true` as an option.
Note that you have to manually install `uglify-es` in order for this to work. Only version 3 is supported.
Example:
Note that if you are using Node 8.3.0 or newer, there are no performance gains from using Uglify. See https://www.nearform.com/blog/node-js-is-getting-a-new-v8-with-turbofan/
```javascript
const stringify = fastJson({
title: 'Example Schema',
type: 'object',
properties: {
id: {
type: 'integer'
}
}
}, { uglify: true })
// stringify is now minified code
console.log(stringify({ some: 'object' })) // '{"some":"object"}'
```
#### Nullable
According to the [Open API 3.0 specification](https://swagger.io/docs/specification/data-models/data-types/#null), a value that can be null must be declared `nullable`.
##### Nullable object
```javascript
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`
## Caveat
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, it's recommended against
allowing user input to directly supply a schema.
It can't be guaranteed that allowing user input for the schema couldn't feasibly expose an attack
vector.
## Acknowledgements
This project was kindly sponsored by [nearForm](http://nearform.com).
## License
MIT