What changed in MobX from 6 to 7

Patch Changes

• 6f002b8e543ca39d3ee23681e28848d7f1724fae #4696 Thanks @gesposito! - perf: add a node export condition that routes Node and Bun to the existing dist/index.js entry, which picks the prebaked development or production CJS build once at require time. importing mobx in Node no longer executes the env-agnostic dist/mobx.mjs with per-call NODE_ENV checks, and mixing import and require in one Node app now yields a single mobx instance.

Patch Changes

• ec1b708026c1578e1c1f6c7bd90be18b26f7ce85 #4695 Thanks @gesposito! - perf: evaluate NODE_ENV once at module scope in the env-agnostic esm bundles (dist/<pkg>.esm.js and dist/<pkg>.mjs) instead of at every __DEV__ call site. process.env is an exotic object in Node, so each check performed a real environment lookup on hot paths; consumers that execute these files as-is (Node ESM, vitest, SSR) see roughly 10x faster observable writes in dev mode. All env-set artifacts and bundler output are unchanged.

• 6d8b5fa6c7596d7e5b16e55e3121d81c0c5bb21a #4672 Thanks @chatman-media! - Fix ObservableSet.replace emitting spurious delete/add events (and triggering reactions) for values that are unchanged. It now only fires delete for removed values and add for newly added ones, mirroring ObservableMap.replace.

  Note: because replace no longer clears and re-adds every value, the iteration order after replace changes in a (subtle but observable) way. Surviving values now keep their original relative position and newly added values are appended, instead of the whole set being reordered to match the argument. For example, set(["a", "b", "c"]).replace(["d", "b", "a"]) previously iterated as d, b, a, and now iterates as a, b, d. This is arguably the more correct behavior (unchanged values are genuinely unchanged), but if you relied on replace reordering the set to match its argument, you may need to adjust.

Patch Changes

• 9444c624b957b489875e1c6b45deb290034ce4e9 #4694 Thanks @mrpmohiburrahman! - fix: onBecomeObserved is now called for the dependencies of a computed that becomes observed while serving a cached value. Previously, observation only cascaded when the newly observed computed also happened to recompute, so an observable with a live observer chain up to a running reaction could still report itself as unobserved and never fire its hook.

• 53bb83fdf455a4e606bb721ea10040ded7c57796 #4683 Thanks @gesposito! - perf: fast-path primitives in deepEnhancer. Writing a primitive into a deep observable no longer runs the observable/array/plain-object/Map/Set/function type checks; primitives can never be made observable, so they are returned immediately. Creating an observable array of primitives is ~4x faster, and observable Set/Map writes are ~20-25% faster in the perf suite.

• 030498d6b2bfe4cc27340fed8c706177cb3b28e1 #4684 Thanks @a-y-ibrahim! - Fix a stack overflow ("Maximum call stack size exceeded") that could occur when an onBecomeUnobserved handler disposes a Reaction. Disposing a Reaction re-enters endBatch(), which used to recurse into the same pendingUnobservations drain loop instead of letting the already-running outer loop pick up the newly queued items, causing unbounded stack depth for long enough chains.

• a9086076b9ead1a9c933215bffaa9b57d44f6829 #4681 Thanks @spokodev! - Fix ObservableSet union, intersection and symmetricDifference to return results in receiver order, matching native Set, when the argument is a plain Set.

• c65a4e14cf48b42cb792dc4c64edbcf56234b32d #4682 Thanks @gesposito! - perf: lazily allocate the internal observers_ Set. Atoms and computed values no longer allocate an empty Set upfront; it is created on first observer instead. Most atoms in large stores are never observed, so this saves roughly 160 bytes per unobserved atom (e.g. ~35% lower heap usage when hydrating 50k instances with 10 observable fields each).

Major Changes

• 1926c69f53168619d688f348aa587e8f3ae579ae #4671 Thanks @kubk! - Release MobX 7, mobx-react-lite 5, and mobx-react 10.

  Bundle sizes are down: ESM prod 17.02 KiB gzip -> 13.96 KiB gzip; a minimal tree-shaken example is 10.32 KiB gzip now.

  It removes long-deprecated compatibility paths and keeps the React bindings split between mobx-react-lite for function components and mobx-react for class-component support.

  MobX 7

  MobX 7 is a cleanup release focused on the modern runtime and decorator model.

  • MobX now always uses Proxy-backed observable objects and arrays. The ES5/non-proxy fallback has been removed.
  • configure({ useProxies: ... }) is no longer supported.
  • { proxy: false } options for observable, observable.object, and observable.array are no longer supported.
  • Legacy decorators are no longer supported.
  • Namespaced annotation and comparer properties now use named exports to reduce bundle size:

  Removed API	Replacement
  observable.ref	observableRef
  observable.shallow	observableShallow
  observable.deep	observableDeep
  observable.struct	observableStruct
  computed.struct	computedStruct
  action.bound	actionBound
  flow.bound	flowBound
  comparer.identity	compareIdentity
  comparer.default	compareDefault
  comparer.structural	compareStructural
  comparer.shallow	compareShallow

  • The public trace API and its related runtime support have been removed. Use toJS, getDependencyTree, getObserverTree, spy or mobx-log package for debugging.

  mobx-react-lite 5 and mobx-react 10

  mobx-react-lite 5 and mobx-react 10 require MobX 7 and React 18 or later.

  mobx-react-lite remains the function-component package. mobx-react remains a thin wrapper around mobx-react-lite that adds class component and Stage 3 @observer class decorator support.

  • Keep function-component imports on mobx-react-lite if you do not need class component support.
  • Use mobx-react when you need class components or @observer class decorators.
  • mobx-react-lite supports function components and forwardRef; mobx-react delegates function components to mobx-react-lite and handles classes itself.
  • Remove React batching imports, including the stale React Native batching deep import. React 18+ renderers handle batching.

  The recommended public React binding surface for both packages is:

  • observer
  • Observer
  • useLocalObservable
  • enableStaticRendering
  • isUsingStaticRendering

  The following APIs have been removed from the React binding packages:

  • Provider, inject, and MobXProviderContext; use React.createContext directly.
  • disposeOnUnmount; dispose reactions in componentWillUnmount or return cleanup functions from useEffect.
  • PropTypes; use TypeScript or the regular prop-types package.
  • useObserver; wrap components with observer or use <Observer>.
  • useLocalStore; use useLocalObservable.
  • useAsObservableSource; synchronize values from props into local observable state explicitly.
  • useStaticRendering; use enableStaticRendering.
  • observerBatching, isObserverBatched, batchingForReactDom, batchingOptOut, and batchingForReactNative; remove these imports because React 18+ renderers handle batching.
  • Deprecated observer(fn, { forwardRef: true }); pass an already-created React.forwardRef(...) component to observer instead.
  • Legacy function-component contextTypes handling.

Patch Changes

• 979d267d569734dc3cb969ed1880ed68bb79f1b5 #4677 Thanks @kubk! - Shorten minified error URL to reduce production bundle size.