fast-deep-equal

fast-deep-equal is a tiny, zero-dependency JavaScript library that performs the fastest possible deep equality comparison between two arbitrary values. It recursively walks objects and arrays and compares Date and RegExp instances by value, returning a simple boolean.

Beyond the default ES5-compatible build, an optional ES6 variant adds equality for Maps, Sets, and typed arrays, and a React-aware variant skips the circular _owner property on React elements. It is one of the most widely depended-on packages on npm, used internally by validators, memoization helpers, and UI libraries.

What You Get

• A single equal(a, b) function that returns a boolean for deep structural equality.
• Multiple prebuilt entry points: default (ES5), fast-deep-equal/es6, and React-aware fast-deep-equal/react.
• Correct value comparison for Date and RegExp objects, plus optional Map, Set, and typed-array support in the ES6 build.
• TypeScript type definitions with a type-guard signature (actual is T).
• Zero runtime dependencies and a footprint small enough to inline into any bundle.

Common Use Cases

• Skipping unnecessary re-renders in UI frameworks by comparing previous and next props or state.
• Deduplicating or diffing configuration and API response objects.
• Powering memoization and caching layers that key on structural equality.
• Writing assertions and equality checks inside test helpers and validators.

Under The Hood

Architecture

The entire library is a single recursive equal(a, b) function. It short-circuits on reference identity, then, for two objects sharing a constructor, branches by shape: arrays are compared element-by-element in a reverse-counting loop, RegExps by source/flags, and any object with a custom valueOf/toString by that value, before falling back to key-by-key recursion over Object.keys. NaN is treated as equal to NaN. All builds are generated from one src/index.jst doT template by build.js, which stamps out the ES5 default (index.js), the ES6 variant with Map/Set/typed-array branches, and React variants that skip the circular _owner property.

Tech Stack

Plain ES5 JavaScript with no runtime dependencies. Build tooling is minimal: the dot templating engine generates the distributable files, ESLint enforces style, and TypeScript is used only to type-check the shipped index.d.ts. Tests run on Mocha with nyc for coverage. The published package ships index.js, react.js, es6/, and .d.ts type definitions.

Code Quality

The implementation is a compact, carefully ordered set of type checks with performance-motivated idioms (reverse for loops, early returns). A data-driven spec suite (spec/tests.js, spec/es6tests.js) exercises scalars, nested structures, special objects, Maps, Sets, typed arrays, and React elements, and the project reports coverage via nyc/coveralls. Because the runtime files are template-generated, contributors edit src/index.jst rather than the .js outputs directly.

API Design The developer experience is essentially frictionless: a single default export called with two arguments returns a boolean, and the three entry points (fast-deep-equal, fast-deep-equal/es6, fast-deep-equal/react) let consumers opt into exactly the type support they need. The TypeScript signature doubles as a type guard (actual is T), and the README documents install, usage, and benchmarks concisely. There is no configuration or setup boilerplate.