feat: fast deep object diff with TypeScript types

- diff() returns typed CREATE/UPDATE/DELETE changes with paths
- applyDiff() applies changes immutably (structuredClone)
- Handles objects, arrays, Dates, RegExps, NaN
- 28 tests, zero dependencies, ~1.4KB

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: oferlmntr

.github/FUNDING.yml

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+github: ofershap

.github/workflows/ci.yml

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+name: CI
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [20, 22]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm test
+      - run: npm run build
+      - run: npm run lint

.gitignore

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+node_modules/
+dist/
+coverage/
+*.tsbuildinfo

LICENSE

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+MIT License
+
+Copyright (c) 2026 Ofer Shapira
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# deep-diff-ts
+
+[![npm version](https://img.shields.io/npm/v/deep-diff-ts)](https://www.npmjs.com/package/deep-diff-ts)
+[![npm downloads](https://img.shields.io/npm/dm/deep-diff-ts)](https://www.npmjs.com/package/deep-diff-ts)
+[![license](https://img.shields.io/npm/l/deep-diff-ts)](https://github.com/ofershap/deep-diff-ts/blob/main/LICENSE)
+[![CI](https://github.com/ofershap/deep-diff-ts/actions/workflows/ci.yml/badge.svg)](https://github.com/ofershap/deep-diff-ts/actions/workflows/ci.yml)
+
+Fast deep object diff with full TypeScript types. Zero dependencies. ~1.4KB.
+
+![Demo](assets/demo.gif)
+
+## Install
+
+```bash
+npm install deep-diff-ts
+```
+
+## Usage
+
+```typescript
+import { diff } from "deep-diff-ts";
+
+const oldObj = {
+  name: "Alice",
+  age: 30,
+  tags: ["admin"],
+  config: { theme: "dark" },
+};
+
+const newObj = {
+  name: "Alice",
+  age: 31,
+  tags: ["admin", "editor"],
+  config: { theme: "light" },
+};
+
+const changes = diff(oldObj, newObj);
+// [
+//   { type: "UPDATE", path: ["age"], oldValue: 30, value: 31 },
+//   { type: "CREATE", path: ["tags", 1], value: "editor" },
+//   { type: "UPDATE", path: ["config", "theme"], oldValue: "dark", value: "light" }
+// ]
+```
+
+### Apply Diffs
+
+```typescript
+import { diff, applyDiff } from "deep-diff-ts";
+
+const changes = diff(oldObj, newObj);
+const result = applyDiff(oldObj, changes);
+// result deeply equals newObj
+// oldObj is not mutated
+```
+
+## Diff Types
+
+Each difference has a `type` and `path`:
+
+| Type     | Fields                      | Description            |
+| -------- | --------------------------- | ---------------------- |
+| `CREATE` | `path`, `value`             | Property was added     |
+| `UPDATE` | `path`, `oldValue`, `value` | Property value changed |
+| `DELETE` | `path`, `oldValue`          | Property was removed   |
+
+`path` is an array of keys/indices: `["users", 0, "name"]`
+
+## Supported Types
+
+- Objects (deep recursive)
+- Arrays (element-by-element)
+- Dates (compared by time value)
+- RegExps (compared by string representation)
+- Primitives (string, number, boolean, null, undefined)
+- NaN (correctly handled via `Object.is`)
+
+## API
+
+### `diff(oldObj, newObj)`
+
+Returns `Difference[]` describing all changes from `oldObj` to `newObj`.
+
+### `applyDiff(target, diffs)`
+
+Returns a new object with all diffs applied. Does not mutate the original.
+
+## Other Projects
+
+- [ts-result](https://github.com/ofershap/ts-result) — Rust-style Result<T, E> for TypeScript
+- [ts-nano-event](https://github.com/ofershap/ts-nano-event) — Typed event emitter in <200 bytes
+- [hover-effects](https://github.com/ofershap/hover-effects) — Canvas-based hover effects for images
+
+## License
+
+MIT

assets/demo.gif

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+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+  { ignores: ["dist/", "node_modules/", "coverage/"] },
+  ...tseslint.configs.recommended,
+);