# MobX 7.0.1

- Product: MobX (https://whatsnew.fyi/product/mobx)
- Vendor: MobX
- Date: 2026-08-19
- Version: 7.0.1
- Original notes: https://github.com/mobxjs/mobx/releases/tag/mobx%407.0.1
- Permalink: https://whatsnew.fyi/product/mobx/releases/7.0.1

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

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- **fixed** — onBecomeObserved is now called for the dependencies of a computed that becomes observed while serving a cached value
- **changed** — fast-path primitives in deepEnhancer to improve performance when writing primitives into deep observables
- **fixed** — Fix a stack overflow that could occur when an onBecomeUnobserved handler disposes a Reaction
- **fixed** — ObservableSet union, intersection and symmetricDifference now return results in receiver order when the argument is a plain Set
- **changed** — Lazily allocate the internal observers_ Set to reduce memory usage for unobserved atoms and computed values

###### Patch Changes

-   [`9444c624b957b489875e1c6b45deb290034ce4e9`](https://github.com/mobxjs/mobx/commit/9444c624b957b489875e1c6b45deb290034ce4e9) [#4694](https://github.com/mobxjs/mobx/pull/4694) Thanks [@mrpmohiburrahman](https://github.com/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`](https://github.com/mobxjs/mobx/commit/53bb83fdf455a4e606bb721ea10040ded7c57796) [#4683](https://github.com/mobxjs/mobx/pull/4683) Thanks [@gesposito](https://github.com/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`](https://github.com/mobxjs/mobx/commit/030498d6b2bfe4cc27340fed8c706177cb3b28e1) [#4684](https://github.com/mobxjs/mobx/pull/4684) Thanks [@a-y-ibrahim](https://github.com/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`](https://github.com/mobxjs/mobx/commit/a9086076b9ead1a9c933215bffaa9b57d44f6829) [#4681](https://github.com/mobxjs/mobx/pull/4681) Thanks [@spokodev](https://github.com/spokodev)! - Fix ObservableSet union, intersection and symmetricDifference to return results in receiver order, matching native Set, when the argument is a plain Set.

-   [`c65a4e14cf48b42cb792dc4c64edbcf56234b32d`](https://github.com/mobxjs/mobx/commit/c65a4e14cf48b42cb792dc4c64edbcf56234b32d) [#4682](https://github.com/mobxjs/mobx/pull/4682) Thanks [@gesposito](https://github.com/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).