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August 25, 2022 15:27
July 12, 2023 12:34


Parse JSON without risk of losing numeric information.

import { parse, stringify } from 'lossless-json'

const text = '{"decimal":2.370,"long":9123372036854000123,"big":2.3e+500}'

// JSON.parse will lose some digits and a whole number:
// '{"decimal":2.37,"long":9123372036854000000,"big":null}'
// WHOOPS!!!

// LosslessJSON.parse will preserve all numbers and even the formatting:
// '{"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.


  • 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 via npm:

npm install lossless-json


Parse and stringify

Parsing and stringification works as you're used to:

import { parse, stringify } from 'lossless-json'

const json = parse('{"foo":"bar"}') // {foo: 'bar'}
const text = stringify(json) // '{"foo":"bar"}'


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.

import { parse } from 'lossless-json'

const text = '{"normal":2.3,"long":123456789012345678901,"big":2.3e+500}'
const json = parse(text)

console.log(json.normal.isLosslessNumber) // true
console.log(json.normal.valueOf()) // number, 2.3

// LosslessNumbers can be used as regular numbers
console.log(json.normal + 2) // number, 4.3

// but the following operation will throw an error as it would result in information loss
console.log(json.long + 1)
// throws Error: Cannot safely convert LosslessNumber to number:
//   "123456789012345678901" will be parsed as 123456789012345680000 and lose information


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:

import { parse, isInteger } from 'lossless-json'

// parse integer values into a bigint, and use a regular number otherwise
export function customNumberParser(value) {
  return isInteger(value) ? BigInt(value) : parseFloat(value)

const text = '[123456789123456789123456789, 2.3, 123]'
const json = parse(text, null, customNumberParser)
// output:
// [
//   123456789123456789123456789n, // bigint
//   2.3, // number
//   123n // bigint
// ]

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:

import { parse, isSafeNumber } from 'lossless-json'

function parseAndValidateNumber(value) {
  if (!isSafeNumber(value)) {
    throw new Error(`Cannot safely convert value '${value}' into a number`)

  return parseFloat(value)

// will parse with success if all values can be represented with a number
let json = parse('[1,2,3]', undefined, parseAndValidateNumber)
console.log(json) // [1, 2, 3] (regular numbers)

// will throw an error when some of the values are too large to represent correctly as number
try {
  let json = parse('[1,2e+500,3]', undefined, parseAndValidateNumber)
} catch (err) {
  console.log(err) // throws Error 'Cannot safely convert value '2e+500' into a number'


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:

import { parse, stringify } from 'lossless-json'
import Decimal from 'decimal.js'

const parseDecimal = (value) => new Decimal(value)

const decimalStringifier = {
  test: (value) => Decimal.isDecimal(value),
  stringify: (value) => value.toString()

// parse JSON, operate on a Decimal value, then stringify again
const text = '{"value":2.3e500}'
const json = parse(text, undefined, parseDecimal) // {value: new Decimal('2.3e500')}
const output = {
  // {result: new Decimal('4.6e500')}
  result: json.value.times(2)
const str = stringify(output, undefined, undefined, [decimalStringifier])
// '{"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:

import { parse, stringify } from 'lossless-json'

// stringify a Date as a unique object with a key '$date', so it is recognizable
function customDateReplacer(key, value) {
  if (value instanceof Date) {
    return {
      $date: value.toISOString()

  return value

function isJSONDateObject(value) {
  return value && typeof value === 'object' && typeof value.$date === 'string'

function customDateReviver(key, value) {
  if (isJSONDateObject(value)) {
    return new Date(value.$date)

  return value

const record = {
  message: 'Hello World',
  timestamp: new Date('2022-08-30T09:00:00Z')

const text = stringify(record, customDateReplacer)
// output:
//   '{"message":"Hello World","timestamp":{"$date":"2022-08-30T09:00:00.000Z"}}'

const parsed = parse(text, customDateReviver)
// output:
//   {
//     action: 'create',
//     timestamp: new Date('2022-08-30T09:00:00.000Z')
//   }


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: JSONValue) => 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.



new LosslessNumber(value: number | string) : LosslessNumber


  • .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:

    // a safe number
    console.log(new LosslessNumber('23.4').valueOf())
    // number 23.4
    // a decimal losing insignificant digits
    console.log(new LosslessNumber('0.66666666666666666666667').valueOf())
    // number 0.6666666666666666
    // a large integer
    console.log(new LosslessNumber('9123372036854000123').valueOf())
    // bigint 9123372036854000123
    // a value that will overflow
    console.log(new LosslessNumber('2.3e+500').valueOf())
    // Error: Cannot safely convert to number: the value '2.3e+500' would overflow and become Infinity
    // a value that will underflow
    console.log(new LosslessNumber('2.3e-500').valueOf())
    // 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.


  • {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:

    isSafeNumber('1.55e3') // true
    isSafeNumber('2e500') // false
    isSafeNumber('2e-500') // false
    isSafeNumber('9123372036854000123') // false
    isSafeNumber('0.66666666666666666667') // false
    isSafeNumber('9123372036854000123', { approx: true }) // false
    isSafeNumber('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:

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

    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.


Similar libraries:


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


Run linting:

npm run lint

Fix linting issues automatically:

npm run format


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

npm run benchmark

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


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.


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


Released under the MIT license.