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  • Fixed that calling "createObservableInstanceIfNeeded" would execute an action, even if the function immediately returned. (significant since the extraneous actions would pollute the mobx dev-tools on mere accesses, eg. by ComplexType.prototype.getValue) Fixes #1421


  • Fix for flow typings. This means that now using flows requires at least TypeScript v3.6 and that castFlowReturn becomes deprecated.
  • Fix for empty models / models with all properties set to optional being able to take any value in TypeScript through #1269 by @xaviergonz.


  • Made it possible to force full run-time type-checking (for better error messages) in production builds by setting ENABLE_TYPE_CHECK=true as environment variable. Fixes #1332 through #1337 by @OverseePublic
  • Fixed an issue where doesn't behave correctly for types that has snapshot processors. Fixes #1321 through #1323 by @Tucker-Eric
  • Changed the implementation of the internal STNValue type, to fix TS 3.5.3 compatibility. If somebody notices regressions in the TypeScript integration, please report. Fixes #1343, #1307
  • Added acceptsUndefined as option for safeReference so it is more suitable to be used inside arrays/maps, through #1245 by @xaviergonz.


  • Fixed a regression with atomic middleware with async flows #1250.
  • Added filter function to recordActions to filter out recording some actions. Also added recording and resume methods.
  • Added getRunningActionContext() to get the currently executing MST action context (if any). Also added the action context helper functions isActionContextChildOf() and isActionContextThisOrChildOf.
  • Reduced type nesting to avoid Typescript 3.4 errors about infinite types. Sadly due to this change types.create is no longer smart enough in TS to know if skipping the snapshot parameter is valid or not. Through #1251 by @xaviergonz.


  • Fixed Instance<typeof variable> not giving the proper type in Typescript when the type included both objects and primitives.
  • Through PR #1196 by @xaviergonz
    • Added createActionTrackerMiddleware2, a more easy to use version of the first one, which makes creating middlewares for both sync and async actions more universal.
    • Added an optional filter to recordPatches to be able to skip recording certain patches.
    • atomic now uses the new createActionTrackerMiddleware2.
    • UndoManager fixes and improvements:
      • Uses the new createActionTrackerMiddleware2.
      • Added clearUndo and clearRedo to only clear those.
      • Added undoLevels and redoLevels to know how many undo/redo actions are available.
      • Made undo manager actions atomic, so they won't actually do any partial changes if for some reason they fail.
      • Fix for withoutUndo so it will only skip recording what is inside, not the whole action - fixes #1195.


  • Added more output formats for the library (common-js minified version and umd minified version). Note that now the umd version will be the development version while the new umd.min version will be the production version. This change is to keep it in sync with the parent mobx package. Also the npm package is now leaner since it mistakenly included separatedly compiled js files and source maps.


  • Fixed a regression with getEnv sometimes not returning the proper environment.
  • Fixed an issue where map.put would not work with snapshots of types with an optional id #1131 through #1226 by @xaviergonz.


  • Added TypeOfValue<typeof variable> to extract the type of a complex (non primitive) variable in Typescript.
  • Fixed some Typescript issues with optional arrays #1218 through #1229 by @xaviergonz
  • Added getNodeId to get the internal unique node id for an instance #1168 through #1225 by @xaviergonz
  • Fixed nodes being pop/shift/splice from an array not getting properly destroyed through #1205 by @xaviergonz. Not that this means that in order to access the returned dead nodes data without getting a liveliness error/warning then the returned dead nodes have to be either cloned (clone) or their snapshots (getSnapshot) have to be used first.


  • Added an optional third argument to types.optional that allows to set alternative optional values other than just undefined through #1192 by @xaviergonz
  • Fixed detaching arrays/maps killing their children #1173 through #1175 by @xaviergonz
  • Added types.snapshotProcessor #947 through #1165 by @xaviergonz. This feature will eventually deprecate postProcessSnapshot and preProcessSnapshot from models in a next major version.
  • Performance improvement for event handlers so they consume less RAM through #1160 by @xaviergonz
  • Make liveliness errors give more info to trace their cause #1142 through #1147 by @xaviergonz


  • Fixed a regression regarding json paths not being correctly rooted to the base #1128 through #1146 by @xaviergonz


  • Fixed mobx 5.9.0 compatibility #1143 through #1144 by @xaviergonz
  • Made liveliness checking in warn mode log an error so the stack trace can be seen #1142 through #1145 by @xaviergonz
  • Fixed JSON path escaping, where '/' and '~' were incorrectly being encoded/decoded as '~0' and '~1' rather than '~1' and '~0'. Also fixed empty keys not being handled correctly by JSON patches #1128. Fixed through #1129 by @xaviergonz


  • Fix for safeReference doesn't work when multiple nodes reference a single reference that gets deleted #1115 through #1121 by @xaviergonz
  • Little fix for castFlowReturn not typecasting the promise to its actual result.
  • Made isAlive(node) reactive, so it can be reacted upon through #1100 by @xaviergonz
  • Fix for unaccessed nodes not unregistering their identifiers #1112 through #1113 by @xaviergonz
  • Added clear() to UndoManager middleware through #1118 by @chemitaxis


  • TypeScript 3.0 or later is now required when using TypeScript. This brings some improvements:
    • flow arguments and return types are now correctly inferred automatically. One exception is when the last return of a flow is a Promise. In these cases castFlowReturn(somePromise) needs to be used so the return type can be inferred properly.
    • create method is now smart enough to warn when no snapshot argument is provided on types that have some mandatory properties.
  • Added setLivelinessChecking and getLivelinessChecking, the old setLivelynessChecking will eventually be deprecated.
  • Added onInvalidated option for references and types.safeReference (see readme) through #1091 by @xaviergonz
  • Added tryReference and isValidReference to use references that might be no longer pointing to any nodes in a safe way through #1087 by @xaviergonz
  • Readded IComplexType for backwards compatibility.


  • Fixed non-initialized nodes not being destroyed #1080 through #1082 by @k-g-a
  • Fixed a memory leak in createActionTrackingMiddleware when using flow #1083 through #1084 by @robinfehr


  • Added castToSnapshot/castToReferenceSnapshot methods for TypeScript and fixed some TypeScript typings not being properly detected when using SnapshotIn types through #1074 by @xaviergonz
  • Fixed redux middleware throwing an error when a flow is called before it is connected #1065 through #1079 by @mkramb and @xaviergonz
  • Made addDisposer return the passed disposer through #1059 by @xaviergonz


  • Fixed references to nodes being broken after the node was replaced #1052, plus speed up of reference resolving when using IDs through #1053 by @xaviergonz


  • Middleware events now also contain allParentIds (chain of causing ids, from root until (excluding) current)
  • Improved redux dev tools integration, now supporting flows and showing action chains through #1035 based on a fix by @bourquep


  • Made type Typescript compilation when 'declarations' is set to true + type completion faster thanks to some type optimizations through #1043 by @xaviergonz
  • Fix for array reconciliation of union types with ids #1045 through #1047 by @xaviergonz
  • Fixed bug where the eager option for the union type defaulted to true when no options argument was passed but false when it was passed. Now they both default to true when not specified. Fixed through #1046 by @xaviergonz


  • Fix for afterCreate/afterAttach sometimes throwing an exception when a node was created as part of a view/computed property #967 through #1026 by @xaviergonz. Note that this fix will only work if your installed peer mobx version is >= 4.5.0 or >= 5.5.0
  • Fix for cast method being broken in Typescript 3.1.1 through #1028 by @xaviergonz




  • Further improvements for Typescript support for enumeration by @xaviergonz
  • Smaller generated .d.ts files through #990 by @xaviergonz
  • Fix for exception when destroying children of types.maybe through #985 by @dsabanin


  • Fixed incorrect typing generation for mst-middlewares #979


  • Fixes for the reconciliation algorithm of arrays #928 through #960 by @liuqiang1357
  • Better Typescript support for enumeration, compose, union, literal and references by @xaviergonz
  • Updated dependencies to latest versions by @xaviergonz
  • [Internal] Cleanup 'createNode' and related codepaths through #962 by @k-g-a





  • Fixed issue where snapshot post-processors where not always applied. Fixes #926, #961, through #959 by @k-g-a


  • Fixed re-adding the same objects to an array. Fixes #928 through #949 by @Krivega


  • Introduced types.integer! By @jayarjo through #935
  • Improved typescript typings, several fixes to the type system. Awesome contribution by @xaviergonz through #937 and #945. Fixes #922, #930, #932, #923
  • Improved handling of types.late

3.0.1 (retracted)


Welcome to MobX-state-tree! This version introduces some breaking changes, but nonetheless is an recommended upgrade as all changes should be pretty straight forward and there is no reason anymore to maintain the 2.x range (3.0 is still compatible with MobX 4)

Most important changes

MST 3 is twice as fast in initializing trees with half the memory consumption compared to version 2:

Running yarn speedtest on Node 9.3:

Time 24sec 12 sec
Mem 315MB 168MB
Size (min+gzip) 14.1KB 15.0KB

Beyond that, MST 3 uses TypeScript 2.8, which results in more accurate TypeScript support.

The type system has been simplified and improved in several areas. Several open issues around maps and (numeric) keys have been resolved. The frozen type can now be fully typed. See below for the full details.

Also, the 'object has died' exception can be suppressed now. One should still address it, but at least it won't be a show-stopper from now on.

Changes in the type system

  • [BREAKING] types.identifier can no longer be parameterized with either types.string or types.number. So instead of types.identifier() use types.identifier. Identifiers are now always normalized to strings. This reflects what was already happening internally and solves a lot of edge cases. To use numbers as identifiers, types.identifierNumber (instead of types.identifier(types.number)) can be used, which serializes it's snapshot to a number, but will internally work like a string based identifier
  • [BREAKING] types.maybe now serializes to / from undefined by default, as it is more and more the common best practice to don't use null at all and MST follows this practice. Use types.maybeNull for the old behavior (see #830)
  • [BREAKING] types.frozen is now a function, and can now be invoked in a few different ways:
    1. types.frozen() - behaves the same as types.frozen in MST 2.
    2. types.frozen(SubType) - provide a valid MST type and frozen will check if the provided data conforms the snapshot for that type. Note that the type will not actually be instantiated, so it can only be used to check the shape of the data. Adding views or actions to SubType would be pointless.
    3. types.frozen(someDefaultValue) - provide a primitive value, object or array, and MST will infer the type from that object, and also make it the default value for the field
    4. types.frozen<TypeScriptType>() - provide a typescript type, to help in strongly typing the field (design time only)
  • It is no longer necessary to wrap or types.array in types.optional when used in a model type, map and array are now optional by default when used as property type. See #906
  • [BREAKING] postProcessSnapshot can no longer be declared as action, but, like preProcessSnapshot, needs to be defined on the type rather than on the instance.
  • [BREAKING] types.union is now eager, which means that if multiple valid types for a value are encountered, the first valid type is picked, rather then throwing. #907 / #804, dispatcher param => option,

Other improvements

  • [BREAKING] MobX-state-tree now requires at least TypeScript 2.8 when using MST with typescript. The type system has been revamped, and should now be a lot more accurate, especially concerning snapshot types.
  • [BREAKING] map.put will now return the inserted node, rather than the map itself. This makes it easier to find objects for which the identifier is not known upfront. See #766 by k-g-a
  • [BREAKING] The order of firing hooks when instantiating has slighlty changed, as the afterCreate hook will now only be fired upon instantiation of the tree node, which now happens lazily (on first read / action). The internal order in which hooks are fired within a single node has remained the same. See #845 for details
  • Significantly improved the performance of constructing MST trees. Significantly reduced the memory footprint of MST. Big shoutout to the relentless effort by k-g-a to optimize all the things! See #845 for details.
  • Introduced setLivelynessChecking("warn" | "ignore" | "error"), this can be used to customize how MST should act when one tries to read or write to a node that has already been removed from the tree. The default behavior is warn.
  • Improved the overloads of model.compose, see #892 by t49tran
  • Fixed issue where computed properties based on getPath could return stale results, fixes #917
  • Fixed issue where onAction middleware threw on dead nodes when attachAfter option was used
  • Fixed several issues with maps and numeric identifiers, such as #884 and #826

TL,DR Migration guide

  • types.identifier(types.number) => types.identifierNumber
  • types.identifier() and types.identifier(types.string) =>types.identifier
  • types.frozen => types.frozen()
  • types.maybe(x) => types.maybeNull(x)
  • postProcessSnapshot should now be declared on the type instead of as action


  • Added support for MobX 5. Initiative by @jeffberry through #868. Please note that there are JavaScript engine restrictions for MobX 5 (no Internet Explorer, or React Native Android). If you need to target those versions please keep using MobX 4 as peer dependency (MST is compatible with both)
  • Reduced memory footprint with ~10-20%, by k-g-a through #872
  • Fixed issue where undo manager was not working correctly for non-root stores, by marcofugaro trough #875


  • Fixed issue where default values of types.frozen where not applied correctly after apply snapshot. #842 by SirbyAlive. Fixes #643
  • Fixed issue where empty patch sets resulted in in unnecessary history items. #838 by chemitaxis. Fixes #837
  • flows of destroyed nodes can no 'safely' resume. #798 by Bnaya. Fixes #792
  • Made sure the type Snapshot is exposed. #821 by dsabanin
  • Fix: the function parameter was incorrectly typed as non-optional. #851 by abruzzihraig


  • It is now possible to get the snapshot of a node without triggering the postProcessSnapshot hook. See #745 for details. By @robinfehr
  • Introduced getParentOfType and hasParentOfType. See #767 by @k-g-a
  • Fixed issue where running typeCheck accidentally logged typecheck errors to the console. Fixes #781


  • Removed accidental dependency on mobx


  • Fixed issue where middleware that changed arguments wasn't properly picked up. See #732 by @robinfehr. Fixes #731
  • Fixed reassigning to a custom type from a different type in a union silently failing. See #737 by @univerio. Fixes #736
  • Fixed typings issue with TypeScript 2.8. See #740 by @bnaya.
  • Fixed undo manager apply grouped patches in the wrong order. See #755 by @robinfehr. Fixes #754


  • Fixed bidirectional references from nodes to nodes, see #728 by @robinfehr
  • joinJsonPath and splitJsonPath are now exposed as utilities, see #724 by @jjrv
  • Several documentation and example fixes


  • Fixed typings for maps of maps #704 by @xaviergonz
  • Fixed dependency issue in mst-middlewares package


Breaking changes

  • MobX-state-tree now requires MobX 4.0 or higher
  • Identifiers are now internally always normalized to strings. This also means that adding an object with an number identifier to an observable map, it should still be requested back as string. In general, we recommend to always use string based identifiers to avoid confusion.
  • Due to the changes in Mobx 4.0, will return Iterator instead of ObservableArrays. In order to address this issue, wrap the keys with Array.from().



  • It is now possible to create custom primitive(like) types! Implements #673 through #689
  • getIdentifier is now exposed as function, to get the identifier of a model instance (if any). Fixes #674 through #678 by TimHollies
  • Writing middleware has slightly changed, to make it less error prone and more explicit whether a middleware chain should be aborted. For details, see #675 by Robin Fehr
  • It is now possible to configure whether attached middleware should be triggered for the built-in hooks / operations. #653 by Robin Fehr
  • We exposed an api to perform reflection on model instances. #649 by Robin Fehr


  • Fixed a bug where items in maps where not properly reconciled when the put operation was used. Fixed #683 and #672 through #693
  • Fixed issue where trying to resolve a path would throw exceptions. Fixed #686 through #692
  • In non production builds actions and views on models can now be replaced, to simplify mocking. Fixes #646 through #690
  • Fixed bug where tryResolve could leave a node in a corrupt state. #668 by dnakov
  • Fixed typings for TypeScript 2.7, through #667 by Javier Gonzalez
  • Several improvements to error messages


  • Fixed bug where flows didn't properly batch their next ticks properly in actions, significantly slowing processes down. Fixes #563


  • Significantly improved the undo/redo manager. The undo manager now supports groups. See #504 by @robinfehr! See the updated docs for more details.
  • Significantly improved performance, improvements of 20% could be expected, but changes of course per case. See #553
  • Implemented actionLogger middleware, which logs most events for async actions
  • Slightly changed the order in which life cycle hooks are fired. afterAttach will no fire first on the parent, then on the children. So, unlike afterCreate, in afterAttach one can assume in `afterAttach that the parent has completely initialized.


  • 1.2.0 didn't seem to be released correctly...


  • Introduced customizable reference types. See the reference and identifiers section.
  • Introduced model.volatile to more easily declare and reuse volatile instance state. Volatile state can contain arbitrary data, is shallowly observable and, like props, cannot be modified without actions. See model.volatile for more details.



  • Fixed an issue where nodes where not always created correctly, see #534. Should fix #513 and #531.
  • All tests are now run in both PROD and non PROD configurations, after running into some bugs that only occurred in production builds.
  • Some internal optimizations have been applied (and many more will follow). Like having internal leaner node for immutable data. See #474
  • A lot of minor improvements on the docs



  • The concept of process (asynchronous actions) has been renamed to flows. (Mainly to avoid issues with bundlers)
  • We changed to a lerna setup which allows separately distributing middleware and testing examples with more ease
  • Every MST middleware is now shipped in a separate package named mst-middlewares. They are now written in TypeScript and fully transpiled to ES5 to avoid problems with uglifyjs in create-react-app bundling.
  • Introduced createActionTrackingMiddleware, this significantly simplifies writing middleware for common scenarios. Especially middleware that deals with asynchronous actions (flows)
  • Renamed process to flow. Deprecated process.
  • BREAKING As a result some middleware event names have also been changed. If you have custom middlewares this change might affect you. Rename middleware event type prefixes starting with process to now start with flow.


  • Fixed nested maps + environments not working correctly, #447 by @xaviergonz
  • Improved typescript typings for enumerations, up to 50 values are now supported #424 by @danielduwaer


  • Introduced modelType.extend which allows creating views and actions with shared state.



  • Added the middlewares atomic and types TimeTraveller, UndoManager. Check out the docs!
  • Introduced createActionTrackingMiddleware to simplify the creation of middleware that support complex async processes
  • exposed typecheck(type, value) as public api (will ignore environment flags)


  • getEnv will return an empty object instead of throwing when a tree was initialized without environment
  • Fixed issue where patches generated for nested maps were incorrect (#396)
  • Fixed the escaping of (back)slashes in JSON paths (#405)
  • Improved the algorithm that reconcile items in an array (#384)
  • Assigning a node that has an environment to a parent is now allowed, as long as the environment is strictly the same (#387)
  • Many minor documentation improvements. Thanks everybody who created a PR!


No changes


  • BREAKING The redux utilities are no longer part of the core package, but need to be imported from mobx-state-tree/middleware/redux.


Breaking changes

  • BREAKING onAction middleware no longer throws when encountering unserializable arguments. Rather, it serializes a struct like { $MST_UNSERIALIZABLE: true, type: "someType" }. MST Nodes are no longer automatically serialized. Rather, one should either pass 1: an id, 2: a (relative) path, 3: a snapshot
  • BREAKING revertPatch has been dropped. IReversableJsonPatch is no longer exposed, instead use the inverse patches generated by onPatch
  • BREAKING some middleware events have been renamed: process_yield -> process_resume, process_yield_error -> process_resume_error, to make it less confusing how these events relate to yield statements.
  • BREAKING patchRecorder's field patches has been renamed to rawPatches,cleanPatchestopatches, andinversePatches` was added.

New features

  • Introduced decorate(middleware, action) to easily attach middleware to a specific action
  • Handlers passed to onPatch(handler: (patch, inversePatch) => void) now receive as second argument the inverse patch of the emitted patch
  • onAction lister now supports an attachAfter parameter
  • Middleware events now also contain parentId (id of the causing action, 0 if none) and tree (the root of context)


  • ReduxDevTools connection is no longer one step behind #287
  • Middleware is no longer run as part of the transaction of the targeted action
  • Fixed representation of union types in error messages


  • BREAKISH Redefining lifecycle hooks will now automatically compose them, implements #252
  • Added dev-only checks, typecheck will be performed only in dev-mode and top-level API-calls will be checked.
  • The internal types IMiddleWareEvent, IMiddlewareEventType, ISerializedActionCall are now exposed (fixes #315)


  • Object model instances no longer share a prototype.


  • Removed accidental dependency on the codemod


  • BREAKING the syntax to define model types has been updated. See the updated docs or the original proposal:#282, but no worries, theres a codemod! :D
  • BREAKING preProcessSnapshot hook is no longer a normal hook that can be defined as action. Instead, it should be defined on the type using types.model(...).preProcessSnapshot(value => value)
  • BREAKING Asynchronous process should now be defined using process. See this example or the asynchronous action docs.

How to run the codemod?

The codemod is provided as npm package command line tool. It has been written using the TypeScript parser, so it will successfully support either TS or regular JavaScript source files.

To run the codemod, you need to first install it globally by npm install -g mst-codemod-to-0.10. After that, the mst-codemod-to-0.10 command will be available in your command line.

To perform the codemod, you need to call in your command line mst-codemod-to-0.10 followed by the filename you want to codemod. A .bak file with the original source will be created for backup purposes, and the file you provided will be updated to the new syntax! Have fun!

PS: You could also use npx instead of installing the codemod globally! :)


  • Asynchronous actions are now a first class concept in mobx-state-tree. See the docs


  • Introduced types.null and types.undefined
  • Introduced types.enumeration(name?, options)


  • Fix note that a snapshot is compatible when assigning a type to an optional version of itself
  • Fix error when deleting a non existing item from a map #255
  • Now all required TypeScript interfaces are exported in the main mobx-state-tree package #256


Introduced the concept of reverse patches, see #231

  • Introduced the revertPatch operation, that takes a patch or list of patches, and reverse applies it to the target.
  • onPatch now takes a second argument, includeOldValue, defaulting to false, which, if set to true, includes in the patch any value that is being overwritten as result of the patch. Setting this option to true produces patches that can be used with revertPatch
  • patchRecorder now introduces additional fields / methods to be able to reverse apply changes: patchRecorder.cleanPatches, patchRecorder.undo


  • Applying a snapshot or patches will now emit an action as well. The name of the emitted action will be @APPLY_PATCHESresp @APPLY_SNAPSHOT. See #107
  • Fixed issue where same Date instance could'nt be used two times in the same state tree #229
  • Fixed issue with reapplying snapshots to Date field resulting in snapshot typecheck error#233
  • Declaring types.maybe(types.frozen) will now result into an error #224
  • Added support for Mobx observable arrays in type checks #221 (from alessioscalici)


  • BREAKING Removed applyPatches and applyActions. Use applyPatch resp. applyAction, as both will now also accept an array as argument
  • BREAKING unprotect and protect can only be applied at root nodes to avoid confusing scenarios Fixed #180
  • Fixed #141, actions / views are no longer wrapped in dynamically generated functions for a better debugging experience
  • Small improvements to typings, fixed compilation issues with TypeScript 2.4.1.
  • Fixed issues where compose couldn't overwrite getters. #209, by @homura
  • Fixed CDN links in readme
  • Added TodoMVC to the examples section


  • Fixed issue in rollup module bundle


  • Fixed issue in release script, rendering 0.8.0 useless


  • BREAKING Dropped types.extend in favor of types.compose. See #192
  • Introduced the lifecycle hooks preProcessSnapshot and postProcessSnapshot. See #203 / #100
  • Use rollup as bundler #196


  • Introduced the concept of volatile / local state in models. See #168, or docs
  • Fixed issue with with types.identifier(types.number) #191 reported by @boatkorachal
  • Fixed issue with reconciler that affected when node already existed at that key reported by @boatkorachal #191


  • Fixed cannot read property resolve of undefined thanks to @cpunion for reporting, now value of dead nodes will be undefined. #186
  • Fixed [LateType] is not defined thanks to @amir-arad for reporting, when using late as model property type #187
  • Fixed Object.freeze can only be called on Object thanks to @ds300 for reporting, when using MST on a ReactNative environment #189
  • Now the entire codebase is prettier! :D #187


  • Fixed array.remove not working


The type system and internal administration has been refactoring, making the internals both simpler and more flexible. Things like references and identifiers are now first class types, making them much better composable. #152

  • BREAKING References with a predefined lookup path are no longer supported. Instead of that, identifiers are now looked up in the entire tree. For that reasons identifiers now have to be unique in the entire tree, per type.
  • BREAKING resolve is renamed to resolvePath
  • Introduced resolveIdentifier(type, tree, identifier) to find objects by identifier
  • BREAKING types.reference is by default non-nullable. For nullable identifiers, use types.maybe(types.reference(X))
  • Many, many improvements. Related open issues will be updated.
  • BREAKING isMST is renamed to isStateTreeNode


  • Fixed issue with array/maps of union types @abruzzihraig #151
  • Make types.extend support computed attributes @cpunion #169
  • Fixed issue with map of primitive types and applySnapshot @pioh #155
  • Better type declarations for union, up to 10 supported types


  • Fixed issue where arrays where not properly serialized as action argument


  • Improved reporting of, now it returns a fine grained report of why the provided value is not applicable.
[mobx-state-tree] Error while converting [{}] to AnonymousModel[]:
at path "/name" snapshot undefined is not assignable to type: string.
at path "/quantity" snapshot undefined is not assignable to type: number.
  • Fixed support for types.reference in combination with types.late, by @robinfehr


  • BREAKING types.withDefault has been renamed to types.optional
  • BREAKING Array and map types can no longer be left out of snapshots by default. Use optional to make them optional in the snapshot
  • BREAKING Literals no longer have a default value by default (use optional + literal instead)
  • BREAKING Disabled inlining type.model definitions as introduced in 0.5.1; to many subtle issues
  • Improved identifier support, they are no properly propagated through utility types like maybe, union etc
  • Fixed issue where fields where not referred back to default when a partial snapshot was provided
  • Fixed #122: types.identifier now also accepts a subtype to override the default string type; e.g. types.identifier(types.number)


  • Introduced support for lazy evaluating values in withDefault, useful to generate UUID's, timestamps or non-primitive default values
  • It is now possible to define something like Removed in 0.6.0
const Box = types.model({
    point: {
        x: 10,
        y: 10

Where the type of point property is inferred to point: types.withDefault(types.model({ x: 10, y: 10}), () => ({ x: 10, y: 10 }))


  • ** BREAKING ** protection is now enabled by default (#101)
  • ** BREAKING ** it is no longer possible to read values from a dead object. Except through getSnapshot or clone (#102)
  • ** BREAKING ** types.recursive has been removed in favor of types.late
  • Introduced unprotect, to disable protection mode for a certain instance. Useful in afterCreate hooks
  • Introduced types.late. Usage: types.late(() => typeDefinition). Can be used for circular / recursive type definitions, even across files. See test/circular(1|2).ts for an example (#74)


BREAKING types.model no requires 2 parameters to define a model. The first parameter defines the properties, derived values and view functions. The second argument is used to define the actions. For example:

const Todo = types.model("Todo", {
  done: types.boolean,
  toggle() {
    this.done = !this.done;

Now should be defined as:

const Todo = types.model(
    done: types.boolean
    toggle() {
      this.done = !this.done;

It is still possible to define functions on the first object. However, those functions are not considered to be actions, but views. They are not allowed to modify values, but instead should produce a new value themselves.


  • Introduced lifecycle hooks afterCreate, afterAttach, beforeDetach, beforeDestroy, implements #76
  • Introduced the convenience method addDisposer(this, cb) that can be used to easily destruct reactions etc. which are set up in afterCreate. See #76


  • Fix: actions where not bound automatically
  • Improved and simplified the reconciliation mechanism, fixed many edge cases
  • Improved the reference mechanism, fixed many edge cases
  • Improved performance


  • (re) introduced the concept of environments, which can be passed as second argument to .create, and picked up using getEnv


  • Removed primitive type, use a more specific type instead
  • Improved typescript typings of snapshots
  • Added depth parameter to getParent and hasParent
  • Separated the concepts of middleware and serializable actions. It is now possible to intercept, modify actions etc through addMiddleWare. onAction now uses middleware, if it is used, all parameters of actions should be serializable!


  • Introduced the concept of liveliness; if nodes are removed from the the tree because they are replaced by some other value, they will be marked as "died". This should help to early signal when people hold on to references that are not part of the tree anymore. To explicitly remove an node from a tree, with the intent to spawn a new state tree from it, use detach.
  • Introduced the convenience method destroy to remove a model from it's parent and mark it as dead.
  • Introduced the concept of protected trees. If a tree is protected using protect, it can only be modified through action, and not by mutating it directly anymore.


  • Introduced .Type and .SnapshotType to be used with TypeScript to get the type for a model


  • Renamed createFactory to types.model (breaking!)
  • Renamed composeFactory to types.extend (breaking!)
  • Actions should now be declared as name(params) { body }, instead of name: action(function (params) { body}) (breaking!)
  • Models are no longer constructed by invoking the factory as function, but by calling factory.create (breaking!)
  • Introduced identifier
  • Introduced / improved reference
  • Greatly improved typescript support, type inference etc. However there are still limitations as the full typesystem of MST cannot be expressed in TypeScript. Especially concerning the type of snapshots and the possibility to use snapshots as first class value.
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