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Preserves types over JSON, BSON or

typeson.js is tiny. 2.6 kb minified. ~1 kb gzipped.

Example of how a stringified object can look

{foo: "bar"}                    // {"foo":"bar"} (simple types gives plain JSON)
{foo: new Date()}               // {"foo":1464049031538, "$types":{"foo":"Date"}}
{foo: new Set([new Date()])}    // {"foo":[1464127925971], "$types":{"foo":"Set","foo.0":"Date"}}
{foo: {sub: /bar/i}}            // {"foo":{"sub":{"source":"bar","flags":"i"}}, "$types":{"foo.sub":"RegExp"}}
{foo: new Int8Array(3)}         // {"foo":"AAAA", "$types":{"foo":"Int8Array"}}
new Date()                      // {"$":1464128478593, "$types":{"$":{"":"Date"}}} (special format at root)


JSON can only contain strings, numbers, booleans, null, arrays and objects. If you want to serialize other types over HTTP, WebSocket, postMessage() or other channels, this module makes it possible to serialize any type over channels that normally only accept vanilla objects. Typeson adds a metadata property $types to the result that maps each non-trivial property to a type name. (In the case of arrays or encoded primitives, a new object will instead be created with a $ property that can be preserved by JSON.) The type name is a reference to a registered type specification that you need to have the same on both the stringifying and the parsing side.

Type Registry

typeson-registry contains encapsulation rules for standard JavaScript types such as Date, Error, ArrayBuffer, etc. Pick the types you need, use a preset or write your own.

const typeson = new Typeson().register([

or if you want support for all built-in javascript classes:

const typeson = new Typeson().register([

The module typeson-registry/presets/builtin is 1.6 kb minizied and gzipped and adds support 32 builtin JavaScript types: Date, RegExp, NaN, Infinity, -Infinity, Set, Map, ArrayBuffer, DataView, Uint8Array, Int8Array, Uint8ClampedArray, Int16Array, Uint16Array, Int32Array, Uint32Array, Float32Array, Float64Array, Error, SyntaxError, TypeError, RangeError, ReferenceError, EvalError, URIError, InternalError, Intl.Collator, Intl.DateTimeFormat, Intl.NumberFormat, Object String, Object Number and Object Boolean.


  • Node
  • Browser
  • Worker
  • ES5


  • Can stringify custom and standard ES5 / ES6 classes.
  • Produces standard JSON with an additional $types property in case it is needed (or a new object if representing a primitive or array at root).
  • Resolves cyclic references, such as lists of objects where each object has a reference to the list
  • You can register (almost) any type to be stringifiable (serializable) with your typeson instance.
  • Output will be identical to that of JSON.stringify() in case your object doesnt contain special types or cyclic references.
  • Type specs may encapsulate its type in other registered types. For example, ImageData is encapsulated as {array: Uint8ClampedArray, width: number, height: number}, expecting another spec to convert the Uint8ClampedArray. With the builtin preset this means it's gonna be converted to base64, but with the socketio preset, its gonna be converted to an ArrayBuffer that is left as-is and streamed binary over the WebSocket channel!


Since typeson has a synchronous API, it cannot encapsulate and revive async types such as Blob, File or Observable. Encapsulating an async object requires to be able to emit streamed content asynchronically. Remoting libraries could however complement typeson with a streaming channel that handles the emitting of stream content. For example, a remoting library could define a typeson rule that encapsulates an Observable to an id (string or number for example), then starts subscribing to it and emitting the chunks to the peer as they arrive. The peer could revive the id to an observable that when subscribed to, will listen to the channel for chunks destinated to the encapsulated ID.


npm install typeson
// Require typeson. It's an UMD module so you could also use requirejs or plain script tags.
const Typeson = require('typeson');
const typeson = new Typeson().register({
    Date: [
        x => x instanceof Date, // test function
        d => d.getTime(), // encapsulator function
        number => new Date(number) // reviver function
    Error: [
        x => x instanceof Error, // tester
        e => ({name:, message: e.message}), // encapsulator
        data => {
          // reviver
          const e = new Error (data.message);
          return e;
    SimpleClass: SimpleClass // Default rules apply. See "register (typeSpec)"
function SimpleClass (foo) { = foo;
// Encapsulate to a JSON friendly format:
const jsonFriendly = typeson.encapsulate({
    date: new Date(),
    e: new Error("Oops"),
    c: new SimpleClass("bar")
// Stringify using good old JSON.stringify()
const json = JSON.stringify(jsonFriendly, null, 2);
  "date": 1464049031538,
  "e": {
    "name": "Error",
    "message": "Oops"
  "c": {
    "foo": "bar"
  "$types": {
    "date": "Date",
    "e": "Error",
    "c": "SimpleClass"
// Parse using good old JSON.parse()
const parsed = JSON.parse(json);
// Revive back again:
const revived = typeson.revive(parsed);

The above sample separates Typeson.encapsulate() from JSON.stringify(). Could also have used Typeson.stringify().

Environment/Format support

Use with can stream ArrayBuffers as real binary data. This is more efficient than encapsulating it in base64/JSON. Typeson can leave certain types, like ArrayBuffer, untouched, and leave the stringification / binarization part to other libs (use Typeson.encapsulate() and not Typeson.stringify()).

What doesn't do though, is preserve Dates, Errors or your custom types.

So to get the best of two worlds:

  • Register preset 'typeson-registry/presets/socketio' as well as your custom types.
  • Use Typeson.encapsulate() to generate an object ready for socket-io emit()
  • Use Typeson.revive() to revive the encapsulated object at the other end.
const Typeson = require('typeson'),
    presetSocketIo = require('typeson-registry/presets/socketio.js');
const TSON = new Typeson()
        CustomClass: [
            x => x instanceof CustomClass,
            c => {foo:, bar:},
            o => new CustomClass(,
const array = new Float64Array(65536);
array.fill(42, 0, 65536);
const data = {
    date: new Date(),
    error: new SyntaxError("Ooops!"),
    array: array,
    custom: new CustomClass("foo", "bar")
socket.emit('myEvent', TSON.encapsulate(data));

The encapsulate() method will not stringify but just traverse the object and return a simpler structure where certain properties are replaced with a substitute. The resulting object will also have a $types property containing the type metadata.

Packing it up at the other end:

socket.on('myEvent', function (data) {
    const revived = TSON.revive(data);
    // Here we have a true Date, SyntaxError, Float64Array and Custom to play with.

NOTE: Both peers must have the same types registered.

Use with BSON

The BSON format can serialize object over a binary channel. It supports just the standard JSON types plus Date, Error and optionally Function. You can use Typeson to encapsulate and revive other types as well with BSON as bearer. Use it the same way as shown above with

Use with Worker.postMessage()

Web Workers have the onmessage and postMessage() communication channel that has built-in support for transferring structures using the structured clone algorithm. It supports Date, ArrayBufferand many other standard types, but not Errors or your own custom classes. To support Error and custom types over web worker channels, register just the types that are needed (Errors and your custom types), and then use Typeson.encapsulate() before posting a message, and Typeson.revive() in the onmessage callback.


constructor ([options])

new Typeson([options]);

Creates an instance of Typeson, on which you may configure additional types to support, or call encapsulate(), revive(), stringify() or parse() on.


options (optional):
    cyclic?: boolean, // Default true to allow cyclic objects
    encapsulateObserver?: function, // Default no-op
    sync?: true, // Don't force a promise response regardless of type
    throwOnBadSyncType?: true // Default to throw when mismatch with `Typeson.Promise` obtained for sync request or not returned for async
cyclic: boolean

Whether or not to support cyclic references. Defaults to true unless explicitly set to false. If this property is false, the parsing algorithm becomes a little faster and in case a single object occurs on multiple properties, it will be duplicated in the output (as JSON.stringify() would do). If this property is true, several instances of same object will only occur once in the generated JSON and other references will just contain a pointer to the single reference.

encapsulateObserver: object (see description)

For encapsulations/stringifications, this callback will be executed as objects are iterated and types are detected. An observer might be used to build an interface based on the original object taking advantage of serialized values (the replaced property) passed to the observer along the way, even potentially without concern to the actual encapsulated result.

encapsulateObserver is passed an object with the following properties:

  • type - If a type was detected, whether at either the awaitingTypesonPromise or resolvingTypesonPromise stage, this property will indicate the detected type. If this is a regular JSON type, its name ('null', 'boolean', 'number', 'string', 'array', or 'object') will be reported
  • keypath - The keypath at which the observer is reporting.
  • value - The original value found at this stage by the observer. (replaced, on the other hand, can be consulted to obtain any type replacement value.)
  • cyclic - A boolean indicating whether the current state is expecting cyclics. Will be "readonly" if this iteration is due to a recursive replacement.
  • stateObj - The state object at the time of observation.
  • promisesData - The promises array.
  • resolvingTypesonPromise - A boolean indicating whether or not this observation is occurring at the (Typeson) promise stage.
  • awaitingTypesonPromise - Will be true if still awaiting the full resolution; this could be ignored or used to set a placeholder.

The following properties are also present in particular cases:

  • typeDetected - Set to true when a Typeson-detected type is found but no relevant replacer is present to be performed.
  • replacing - Set to true preceding a Typeson-detected type replacement. Use this to get at the original object value before encapsulation
  • clone - If a plain object or array is found or if iterateIn is set, this property holds the clone of that object or array.
  • replaced - This property will be set when a type was detected. This value is useful for obtaining the serialization of types.
  • cyclicKeypath - Will be present if a cyclic object (including array) were detected; refers to the key path of the prior detected object.
  • endIterateIn - Will be true if finishing iteration of in properties.
  • endIterateOwn - Will be true if finishing iteration of "own" properties.
  • endIterateUnsetNumeric - Will be true if finishing iteration of unset numeric properties.
  • end - Convenience property that will be true if endIterateIn, endIterateOwn, or endIterateUnsetNumeric is true.
sync: boolean (Internal property)

Types can utilize Typeson.Promise to allow asynchronous encapsulation and stringification.

When such a type returns a Typeson.Promise, a regular Promise will be returned to the user.

(This property is used internally for ensuring a regular Promise was not intended as the result. Note that its resolved value is also recursively checked for types.)

To ensure that a regular Promise is always returned and thereby to allow the same API to be used regardless of the types in effect, the sync option is set to false by the *Async methods.

Note that this has no bearing on revive/parse since they can construct any object they wish for a return value, including a Promise, a stream, etc.

throwOnBadSyncType: boolean

The default is to throw when an async result is received from a synchronous method or vice versa. This assures you that you are receiving the intended result type.

This option can be set to false, however, to return the raw synchronous result or the promise, allowing you the least unambiguous results (since you can discern whether a returned Promise was the actual result of a revival/parsing or just the inevitable return of using an async method).


const Typeson = require('typeson');
const typeson = new Typeson()
    .register (require('typeson-registry/presets/builtin'));
const tson = typeson.stringify(complexObject);
const obj = typeson.parse(tson);



A map between type identifier and type-rules. Same (object-based) structure as passed to register(). Use this property if you want to create a new Typeson containing all types from another Typeson.

const commonTypeson = new Typeson().register([
const myTypeson = new Typeson().register([
    commonTypeson.types, // Derive from commonTypeson
    myOwnSpecificTypes // Add your extra types

Instace methods

stringify (obj, [replacer], [space], [options])

Initial arguments identical to those of JSON.stringify()

Generates JSON based on the given obj. Applies JSON.stringify() on the result of any relevant replace encapsulators.

If the supplied obj has special types or cyclic references, the produced JSON will contain a $types property on the root upon which type info relies (a map of keypath to type where the keypath is dot-separated; see Typeson.escapeKeyPathComponent on escaping).

The options object argument can include a setting for cyclic which overrides the default or any behavior supplied for this option in the Typeson constructor.

May also return a Promise if a type's replace encapsulator returns Typeson.Promise. See the documentation under Typeson.Promise.

Stringification format

If enabled, the cyclic "type" will be represented as # and cyclic references will be encoded as # plus the path to the referenced object.

If an array or primitive is encoded at root, an object will be created with a property $ and a $types property that is an object with $ as a key and instead of a type string as value, a keypath-type object will be its value (with the empty string indicating the root path).

const TSON = new Typeson().register(require('typeson-registry/types/date'));
TSON.stringify ({date: new Date()});


{"date": 1463667643065, "$types": {"date": "Date"}}

stringifySync (obj, [replacer], [space], [options])

As with stringify but automatically throws upon obtaining a Typeson.Promise return result from a replace encapsulator (as that is expected for asynchronous types).

stringifyAsync (obj, [replacer], [space], [options])

As with stringify but automatically throws upon obtaining a non-Typeson.Promise return result from a replace encapsulator (as only a Typeson.Promise is expected for asynchronous types).

parse (obj, [reviver])

Arguments identical to those of JSON.parse()

Parses Typeson-generated JSON back into the original complex structure again.

Applies JSON.parse() and then any relevant revive methods that are detected.

May also return a Promise if a type's reviver returns Typeson.Promise. See the documentation under Typeson.Promise.

const TSON = new Typeson().register(require('typeson-registry/types/date'));
TSON.parse ('{"date": 1463667643065, "$types": {"date": "Date"}}');

parseSync (obj, [reviver])

As with parse but automatically throws upon obtaining a Typeson.Promise return result from the reviver (as that is expected for asynchronous types).

parseAsync (obj, [reviver])

As with parse but automatically throws upon obtaining a non-Typeson.Promise return result from the reviver (as only a Typeson.Promise is expected for asynchronous types).

encapsulate (obj, [stateObj], [opts])

Encapsulates an object but leaves the stringification part to you. Pass your encapsulated object further to, postMessage(), BSON or IndexedDB.

Applies the replace method on test-matching spec objects. Will return the result regardless of whether it is an asynchronous (indicated by a Typeson.Promise) or synchronous result.

The options object argument can include a setting for cyclic which overrides the default or any behavior supplied for this option in the Typeson constructor.

encapsulateSync (obj, [opts])

As with encapsulate but automatically throws upon obtaining a Typeson.Promise return result from the replacer (as that is expected for asynchronous types).

encapsulateAsync (obj, [opts])

As with encapsulate but automatically throws upon obtaining a non-Typeson.Promise return result from the replacer (as only a Typeson-Promise is expected for asynchronous types).

const encapsulated = typeson.encapsulate(new Date());
const revived = typeson.revive(encapsulated);
assert (revived instanceof Date);

revive / reviveSync / reviveAsync (obj)

Revives an encapsulated object. See encapsulate().

register (typeSpec, opts = {fallback: boolean|number})

If opts.fallback is set, lower priority will be given (the default is that the last registered item has highest priority during match testing). If a number is given, it will be used as the index of the placement.


An object that maps a type-name to a specification of how to test, encapsulate and revive that type.

{TypeName => constructor-function | [tester, encapsulator, reviver] | specObject = {test: function, replace: function, revive: function, testPlainObjects: boolean=false}} or an array of such structures.

Please note that if an array is supplied, the tester (and upon matching, the encapsulator) execute in a last-in, first out order. (Calls to register can set fallback to true to lower the priority of a recent addition.)

this will refer to the specification object.

Subsequent calls to register will similarly be given higher priority so be sure to add catch-all matchers before more precise ones.

If testPlainObjects is set to true, a tester will be checked against plain objects and allow replacements without recursion.


A class (constructor function) that would use default test, encapsulation and revival rules, which is:

  • test: check if x.constructor === constructor-function.
  • replace: copy all enumerable own props into a vanilla object
  • revive: Uses Object.create() to revive the correct type and copies all properties into it.
  • testPlainObjects: false: Tests non-plain objects only.
test (obj : any, stateObj : {ownKeys: boolean, iterateIn: ('array'|'object'), iterateUnsetNumeric: boolean}) : boolean

Function that tests whether an instance is of your type and returns a truthy value if it is.

If the context is iteration over non-"own" integer string properties of an array (i.e., an absent (undefined) item in a sparse array), ownKeys will be set to false. Otherwise, when iterating an object or array, it will be set to true. The default for the stateObj is just an empty object.

If you wish to have exceptions thrown upon encountering a certain type of value, you may leverage the tester to do so.

You may also set values on the state object.

Normally, only the "own" keys of an object will be iterated. Setting iterateIn changes the behavior to iterate all properties "in" the object for cloning (though note that doing so will add a performance cost). The value of iterateIn (as 'array' or 'object') determines what type of object will be created. Normally, 'object' will be more useful as non-array-index properties do not survive stringification on an array.

One special case not covered by iterating all "own" keys or enabling "in" iteration is where one may wish to iterate the keys not "in" the object but still part of it, i.e., the unset numeric indexes of a sparse array (e.g., for the sake of ensuring they are ignored entirely rather than converted to null by a stringify call). Thus encapsulators have the ability to set iterateUnsetNumeric: true on their state object, but note that doing so will add a performance cost.

replace (obj: YourType, stateObj : {ownKeys: boolean, iterateIn: ('array'|'object'), iterateUnsetNumeric: boolean}) : Object

Function that maps your instance to a JSON-serializable object. Can also be called an encapsulator. For the stateObj, see tester. In a property context (for arrays or objects), returning undefined will prevent the addition of the property.

See the tester for a discussion of the stateObj.

Note that replacement results will themselves be recursed for state changes and type detection.

replaceAsync (obj: YourType, stateObj : {ownKeys: boolean, iterateIn: ('array'|'object'), iterateUnsetNumeric: boolean}) : Typeson.Promise

Expected to return a Typeson.Promise which resolves to the replaced value. See replace.

revive (obj: Object) : YourType

Function that maps your JSON-serializable object into a real instance of your type. In a property context (for arrays or objects), returning undefined will prevent the addition of the property. To explicitly add undefined, see Typeson.Undefined.

const typeson = new Typeson();
function CustomType(foo) { = foo;
  // simple style - provide just a constructor function.
  // This style works for any trivial js class without hidden closures.
  CustomType: CustomType,
  // Date is native and hides it's internal state.
  // We must define encapsulator and reviver that always works.
  Date: [
    x => x instanceof Date, // tester
    date => date.getTime(), // encapsulator
    obj => new Date(obj)    // reviver
  RegExp: [
    x = x instanceof RegExp,
    re => [re.source, re.flags],
    a => new RegExp (a[0], a[1])
    ct: new CustomType("hello"),
    d: new Date(),
// {"ct":{"foo":"hello"},"d":1464049031538,"r":["foo","gi"],$types:{"ct":"CustomType","d":"Date","r":"RegExp"}}
reviveAsync (obj: Object) : YourType

Expected to return a Typeson.Promise which resolves to the revived value. See revive.

specialTypeNames (obj, [stateObj], [opts])

This method returns an array of the unique Typeson type names. To return all type names including JSON type names or duplicates, use an encapsulateObserver.

rootTypeName (obj, [stateObj], [opts])

This method returns a single type name string of the supplied object at root: a Typeson type if present or a JSON type otherwise. This method avoids iterating whole object/array structures.

Class methods

Typeson.Undefined class

During encapsulation, undefined will not be set for property values, of objects or arrays (including sparse ones and replaced values) (undefined will be converted to null if stringified anyways). During revival, however, since undefined is also used in this context to indicate a value will not be added, if you wish to have an explicit undefined added, you can return new Typeson.Undefined() to ensure a value is set explicitly to undefined.

This distinction is used by the undefined type in typeson-registry to allow reconstruction of explicit undefined values (and its sparseUndefined type will ensure that sparse arrays can be reconstructed).

Typeson.Promise class

If you have a type which you wish to have resolved asynchronously, you can can return a Typeson.Promise (which works otherwise like a Promise) and call its first supplied argument (resolve) when ready.

The reason we expect this class to be used here instead of regular Promises as types might wish to serialize them in their own manner (or perhaps more likely, to be able to throw when encountering them if they are not expected).

function MyAsync (prop) {
    this.prop = prop;
const typeson = new Typeson({sync: false}).register({
    myAsyncType: [
        function (x) { return x instanceof MyAsync;},
        function (o) {
            return new Typeson.Promise(function (resolve, reject) {
                setTimeout(function () { // Do something more useful in real code
                }, 800);
        function (data) {
            return new MyAsync(data);
const mya = new MyAsync(500);
return typeson.stringify(mya).then(function (result) {
    const back = typeson.parse(result, null, {sync: true});
    console.log(back.prop); // 500


A simple utility for getting the former [[Class]] internal slot of an object (i.e., The string between [Object and ] as returned from Object.prototype.toString) or what is known in HTML as the "class string".

Since Symbol.toStringTag can set the value for other objects and is defined by JavaScript itself, we use that within the method name.

The method can be used for cross-frame detection of your objects as well as objects associated with all platform objects (i.e., non-callback interfaces or DOMExceptions) tied to WebIDL (such as the interfaces in HTML). The platform object's identifier (i.e., the interface name) is, per the WebIDL spec, the string to be returned.

Although it is unfortunately not immune to forgery, it may in some cases be more appealing than (or usable in addition to) duck typing so this tiny utility is bundled for convenience.

Typeson.hasConstructorOf (objWithPrototypeConstructor, classToCompare: constructor or null) : boolean

Another approach for class comparisons involves checking a constructor function and comparing its toString. This is required for some classes which otherwise do not define toStringTags which differ from other objects. The first argument will be an object to check (whose prototoype will be searched for a constructor property) whereas the second is a class constructor to compare.

If no valid constructor is found, false will be returned unless null was supplied as the classToCompare in which case true will be returned.

Typeson.isObject (val)

Simple but frequently-needed type-checking utility for val && typeof val === 'object' to avoid null being treated as an object.

Typeson.isPlainObject (val)

Checks for a simple non-inherited object. Adapted from jQuery's isPlainObject.

Typeson.isUserObject (val)

Allows for inherited objects but ensures the prototype chain inherits from Object (or null).

Typeson.isThenable (val, catchCheck=boolean)

Checks whether an object is "thenable" (usable as a promise). If the second argument is supplied as true, it will also ensure it has a catch method. A regular Promise or Typeson.Promise will return true.

Typeson.escapeKeyPathComponent (unescapedKeyPathComponent)

Escapes a component of a key path.

Dots in property names are escaped as ~1, and the tilde escape character is itself escaped as ~0.

Typeson.unescapeKeyPathComponent (escapedKeyPathComponent)

Unescapes a key path component. See Typeson.escapeKeyPathComponent.

Typeson.getByKeyPath (obj, keyPath)

Retrieves a value pointed to by a key path on an object.

Typeson.getJSONType (obj)

Utility that returns 'null', 'boolean', 'number', 'string', 'array', or 'object' depending on JSON type.

Class properties


Set to the following array of JSON type names.

['null', 'boolean', 'number', 'string', 'array', 'object']

Finding types and groups of types

typeson-registry contains ready-to-use types and presets to register with your Typeson instances.