lossless-json

Parse JSON without risk of losing numeric information.

1import { parse, stringify } from 'lossless-json'
2
3const text = '{"decimal":2.370,"long":9123372036854000123,"big":2.3e+500}'
4
5// JSON.parse will lose some digits and a whole number:
6console.log(JSON.stringify(JSON.parse(text)))
7// '{"decimal":2.37,"long":9123372036854000000,"big":null}'
8// WHOOPS!!!
9
10// LosslessJSON.parse will preserve all numbers and even the formatting:
11console.log(stringify(parse(text)))
12// '{"decimal":2.370,"long":9123372036854000123,"big":2.3e+500}'

The following in-depth article explains what happens there: Why does JSON.parse corrupt large numbers and how to solve this?

How does it work? The library works exactly the same as the native JSON.parse and JSON.stringify. The difference is that lossless-json preserves information of big numbers. lossless-json parses numeric values not as a regular number but as a LosslessNumber, a lightweight class which stores the numeric value as a string. One can perform regular operations with a LosslessNumber, and it will throw an error when this would result in losing information.

When to use? If you have to deal with JSON data that contains long values for example, coming from an application like C++, Java, or C#. The trade-off is that lossless-json is slower than the native JSON.parse and JSON.stringify functions, so be careful when performance is a bottleneck for you.

Features:

• No risk of losing numeric information when working with big numbers.
• Maintain the formatting of numbers.
• Parse error on duplicate keys.
• Built-in support for bigint.
• Built-in support for Date (turned off by default).
• Customizable: parse numeric values into any data type, like BigNumber, bigint, number, or a mix of them.
• Compatible with the native, built-in JSON.parse and JSON.stringify.
• Helpful error messages when parsing invalid JSON.
• Works in browsers and node.js.
• Comes with TypeScript typings included.
• Modular: ES module functions, only load and bundle what you use.
• The full bundle is less than 4kB in size when minified and gzipped.

Install

Install via npm:

npm install lossless-json

Use

Parse and stringify

Parsing and stringification works as you're used to:

1import { parse, stringify } from 'lossless-json'
2
3const json = parse('{"foo":"bar"}') // {foo: 'bar'}
4const text = stringify(json) // '{"foo":"bar"}'

LosslessNumbers

Numbers are parsed into a LosslessNumber, which can be used like a regular number in numeric operations. Converting to a number will throw an error when this would result in losing information due to truncation, overflow, or underflow.

1import { parse } from 'lossless-json'
2
3const text = '{"normal":2.3,"long":123456789012345678901,"big":2.3e+500}'
4const json = parse(text)
5
6console.log(json.normal.isLosslessNumber) // true
7console.log(json.normal.valueOf()) // number, 2.3
8
9// LosslessNumbers can be used as regular numbers
10console.log(json.normal + 2) // number, 4.3
11
12// but the following operation will throw an error as it would result in information loss
13console.log(json.long + 1)
14// throws Error: Cannot safely convert LosslessNumber to number:
15//   "123456789012345678901" will be parsed as 123456789012345680000 and lose information

BigInt

JavaScript natively supports bigint: big integers that can hold a large number of digits, instead of the about 15 digits that a regular number can hold. It is a typical use case to want to parse integer numbers into a bigint, and all other values into a regular number. This can be achieved with a custom numberParser:

1import { parse, isInteger } from 'lossless-json'
2
3// parse integer values into a bigint, and use a regular number otherwise
4export function customNumberParser(value) {
5  return isInteger(value) ? BigInt(value) : parseFloat(value)
6}
7
8const text = '[123456789123456789123456789, 2.3, 123]'
9const json = parse(text, null, customNumberParser)
10// output:
11// [
12//   123456789123456789123456789n, // bigint
13//   2.3, // number
14//   123n // bigint
15// ]

You can adjust the logic to your liking, using utility functions like isInteger, isNumber, isSafeNumber. The number parser shown above is included in the library and is named parseNumberAndBigInt.

Validate safe numbers

If you want parse a json string into an object with regular numbers, but want to validate that no numeric information is lost, you write your own number parser and use isSafeNumber to validate the numbers:

1import { parse, isSafeNumber } from 'lossless-json'
2
3function parseAndValidateNumber(value) {
4  if (!isSafeNumber(value)) {
5    throw new Error(`Cannot safely convert value '${value}' into a number`)
6  }
7
8  return parseFloat(value)
9}
10
11// will parse with success if all values can be represented with a number
12let json = parse('[1,2,3]', undefined, parseAndValidateNumber)
13console.log(json) // [1, 2, 3] (regular numbers)
14
15// will throw an error when some of the values are too large to represent correctly as number
16try {
17  let json = parse('[1,2e+500,3]', undefined, parseAndValidateNumber)
18} catch (err) {
19  console.log(err) // throws Error 'Cannot safely convert value '2e+500' into a number'
20}

BigNumbers

To use the library in conjunction with your favorite BigNumber library, for example decimal.js. You have to define a custom number parser and stringifier:

1import { parse, stringify } from 'lossless-json'
2import Decimal from 'decimal.js'
3
4const parseDecimal = (value) => new Decimal(value)
5
6const decimalStringifier = {
7  test: (value) => Decimal.isDecimal(value),
8  stringify: (value) => value.toString()
9}
10
11// parse JSON, operate on a Decimal value, then stringify again
12const text = '{"value":2.3e500}'
13const json = parse(text, undefined, parseDecimal) // {value: new Decimal('2.3e500')}
14const output = {
15  // {result: new Decimal('4.6e500')}
16  result: json.value.times(2)
17}
18const str = stringify(output, undefined, undefined, [decimalStringifier])
19// '{"result":4.6e500}'

Reviver and replacer

The library is compatible with the native JSON.parse and JSON.stringify, and also comes with the optional reviver and replacer arguments that allow you to serialize for example data classes in a custom way. Here is an example demonstrating how you can stringify a Date in a different way than the built-in reviveDate utility function.

The following example stringifies a Date as an object with a $date key instead of a string, so it is uniquely recognizable when parsing the structure:

1import { parse, stringify } from 'lossless-json'
2
3// stringify a Date as a unique object with a key '$date', so it is recognizable
4function customDateReplacer(key, value) {
5  if (value instanceof Date) {
6    return {
7      $date: value.toISOString()
8    }
9  }
10
11  return value
12}
13
14function isJSONDateObject(value) {
15  return value && typeof value === 'object' && typeof value.$date === 'string'
16}
17
18function customDateReviver(key, value) {
19  if (isJSONDateObject(value)) {
20    return new Date(value.$date)
21  }
22
23  return value
24}
25
26const record = {
27  message: 'Hello World',
28  timestamp: new Date('2022-08-30T09:00:00Z')
29}
30
31const text = stringify(record, customDateReplacer)
32console.log(text)
33// output:
34//   '{"message":"Hello World","timestamp":{"$date":"2022-08-30T09:00:00.000Z"}}'
35
36const parsed = parse(text, customDateReviver)
37console.log(parsed)
38// output:
39//   {
40//     action: 'create',
41//     timestamp: new Date('2022-08-30T09:00:00.000Z')
42//   }

API

parse(text [, reviver [, parseNumber]])

The LosslessJSON.parse() function parses a string as JSON, optionally transforming the value produced by parsing.

• @param {string} text The string to parse as JSON. See the JSON object for a description of JSON syntax.
• @param {(key: string, value: unknown) => unknown} [reviver] If a function, prescribes how the value originally produced by parsing is transformed, before being returned.
• @param {function(value: string) : unknown} [parseNumber] Pass an optional custom number parser. Input is a string, and the output can be any numeric value: number, bigint, LosslessNumber, or a custom BigNumber library. By default, all numeric values are parsed into a LosslessNumber.
• @returns {unknown} Returns the Object corresponding to the given JSON text.
• @throws Throws a SyntaxError exception if the string to parse is not valid JSON.

stringify(value [, replacer [, space [, numberStringifiers]]])

The LosslessJSON.stringify() function converts a JavaScript value to a JSON string, optionally replacing values if a replacer function is specified, or optionally including only the specified properties if a replacer array is specified.

• @param {unknown} value The value to convert to a JSON string.
• @param {((key: string, value: unknown) => unknown) | Array.<string | number>} [replacer] A function that alters the behavior of the stringification process, or an array with strings or numbers that serve as a whitelist for selecting the properties of the value object to be included in the JSON string. If this value is null or not provided, all properties of the object are included in the resulting JSON string.
• @param {number | string | undefined} [space] A string or number that is used to insert white space into the output JSON string for readability purposes. If this is a number, it indicates the number of space characters to use as white space. Values less than 1 indicate that no space should be used. If this is a string, the string is used as white space. If this parameter is not provided (or is null), no white space is used.
• @param {Array<{test: (value: unknown) => boolean, stringify: (value: unknown) => string}>} [numberStringifiers] An optional list with additional number stringifiers, for example to serialize a BigNumber. The output of the function must be valid stringified JSON number. When undefined is returned, the property will be deleted from the object. The difference with using a replacer is that the output of a replacer must be JSON and will be stringified afterwards, whereas the output of the numberStringifiers is already stringified JSON.
• @returns {string | undefined} Returns the string representation of the JSON object.
• @throws Throws a SyntaxError when one of the numberStringifiers does not return valid output.

LosslessNumber

Construction

new LosslessNumber(value: number | string) : LosslessNumber

Methods

• .valueOf(): number | bigint Convert the LosslessNumber into a regular number or bigint. A number is returned for safe numbers and decimal values that only lose some insignificant digits. A bigint is returned for large integer numbers. An Error is thrown for values that will overflow or underflow. Examples:

  1// a safe number
  2console.log(new LosslessNumber('23.4').valueOf())
  3// number 23.4
  4
  5// a decimal losing insignificant digits
  6console.log(new LosslessNumber('0.66666666666666666666667').valueOf())
  7// number 0.6666666666666666
  8
  9// a large integer
  10console.log(new LosslessNumber('9123372036854000123').valueOf())
  11// bigint 9123372036854000123
  12
  13// a value that will overflow
  14console.log(new LosslessNumber('2.3e+500').valueOf())
  15// Error: Cannot safely convert to number: the value '2.3e+500' would overflow and become Infinity
  16
  17// a value that will underflow
  18console.log(new LosslessNumber('2.3e-500').valueOf())
  19// Error: Cannot safely convert to number: the value '2.3e-500' would underflow and become 0

  Note that you can implement your own strategy for conversion by just getting the value as string via .toString(), and using util functions like isInteger, isSafeNumber, getUnsafeNumberReason, and toSafeNumberOrThrow to convert it to a numeric value.

• .toString() : string Get the string representation of the lossless number.

Properties

• {boolean} .isLosslessNumber : true Lossless numbers contain a property isLosslessNumber which can be used to check whether some variable contains LosslessNumber.

Utility functions

• isInteger(value: string) : boolean Test whether a string contains an integer value, like '2300' or 10.

• isNumber(value: string) : boolean Test whether a string contains a numeric value, like '2.4' or '1.4e+3'.

• isSafeNumber(value: string, config?: { approx: boolean }): boolean Test whether a string contains a numeric value which can be safely represented by a JavaScript number without losing any information. Returns false when digits would be truncated of an integer or decimal, or when the number would overflow or underflow. When passing { approx: true } as config, the function will be less strict and allow losing insignificant digits of a decimal value. Examples:

  1isSafeNumber('1.55e3') // true
  2isSafeNumber('2e500') // false
  3isSafeNumber('2e-500') // false
  4isSafeNumber('9123372036854000123') // false
  5isSafeNumber('0.66666666666666666667') // false
  6isSafeNumber('9123372036854000123', { approx: true }) // false
  7isSafeNumber('0.66666666666666666667', { approx: true }) // true

• toSafeNumberOrThrow(value: string, config?: { approx: boolean }) : number Convert a string into a number when it is safe to do so, otherwise throw an informative error.

• getUnsafeNumberReason(value): UnsafeNumberReason | undefined When the provided value is an unsafe number, describe what the reason is: overflow, underflow, truncate_integer, truncate_float. Returns undefined when the value is safe.

• isLosslessNumber(value: unknown) : boolean Test whether a value is a LosslessNumber.

• toLosslessNumber(value: number) : LosslessNumber Convert a number into a LosslessNumber. The function will throw an exception when the number is exceeding the maximum safe limit of 15 digits (hence being truncated itself) or is NaN or Infinity.

• parseLosslessNumber(value: string) : LosslessNumber The default numberParser used by parse. Creates a LosslessNumber from a string containing a numeric value.

• parseNumberAndBigInt(value: string) : number | bigint A custom numberParser that can be used by parse. The parser will convert integer values into bigint, and converts al other values into a regular number.

• reviveDate(key, value) Revive strings containing an ISO 8601 date string into a JavaScript Date object. This reviver is not turned on by default because there is a small risk of parsing a text field that accidentally contains a date into a Date. Whether reviveDate is safe to use depends on the use case. Usage:

  1import { parse, reviveDate } from 'lossless-json'
  2
  3const data = parse('["2022-08-25T09:39:19.288Z"]', reviveDate)
  4// output:
  5// [
  6//   new Date('2022-08-25T09:39:19.288Z')
  7// ]

  An alternative solution is to stringify a Date in a specific recognizable object like {'$date':'2022-08-25T09:39:19.288Z'}, and use a reviver and replacer to turn this object into a Date and vice versa.

Alternatives

Similar libraries:

• https://github.com/jawj/json-custom-numbers
• https://github.com/sidorares/json-bigint
• https://github.com/nicolasparada/js-json-bigint
• https://github.com/epoberezkin/json-source-map

Test

To test the library, first install dependencies once:

npm install

To run the unit tests:

npm test

To build the library and run the unit tests and integration tests:

npm run build-and-test

Lint

Run linting:

npm run lint

Fix linting issues automatically:

npm run format

Benchmark

To run a benchmark to compare the performance with the native JSON parser:

npm run benchmark

(Spoiler: lossless-json is much slower than native)

Build

To build a bundled and minified library (ES5), first install the dependencies once:

npm install

Then bundle the code:

npm run build

This will generate an ES module output and an UMD bundle in the folder ./.lib which can be executed in browsers and node.js and used in the browser.

Release

To release a new version:

$ npm run release

This will:

• lint
• test
• build
• increment the version number
• push the changes to git, add a git version tag
• publish the npm package

To try the build and see the change list without actually publishing:

$ npm run release-dry-run

License

Released under the MIT license.