# MobX 7.0.2

- Product: MobX (https://whatsnew.fyi/product/mobx)
- Vendor: MobX
- Date: 2026-08-19
- Version: 7.0.2
- Original notes: https://github.com/mobxjs/mobx/releases/tag/mobx%407.0.2
- Permalink: https://whatsnew.fyi/product/mobx/releases/7.0.2

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** — Evaluate NODE_ENV once at module scope in env-agnostic ESM bundles instead of at every __DEV__ call site, improving observable write performance by roughly 10x in dev mode for consumers using Node ESM, vitest, or SSR
- **fixed** — Fix ObservableSet.replace emitting spurious delete/add events for unchanged values, now only firing delete for removed values and add for newly added ones
- **changed** — ObservableSet.replace now preserves the iteration order of surviving values instead of reordering the entire set to match the argument, with newly added values appended

###### Patch Changes

-   [`ec1b708026c1578e1c1f6c7bd90be18b26f7ce85`](https://github.com/mobxjs/mobx/commit/ec1b708026c1578e1c1f6c7bd90be18b26f7ce85) [#4695](https://github.com/mobxjs/mobx/pull/4695) Thanks [@gesposito](https://github.com/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`](https://github.com/mobxjs/mobx/commit/6d8b5fa6c7596d7e5b16e55e3121d81c0c5bb21a) [#4672](https://github.com/mobxjs/mobx/pull/4672) Thanks [@chatman-media](https://github.com/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.