Package - fast-equals

fast-equals

Perform blazing fast equality comparisons between two objects, while also allowing for flexibility for various use-cases. It has no dependencies, and is ~2kB when minified and gzipped.

The following types are handled out-of-the-box:

Plain objects (including react elements and Arguments)
Arrays
ArrayBuffer / TypedArray / DataView instances
Date objects
RegExp objects
Map / Set iterables
Promise objects and then-ables
Primitive wrappers (new Boolean() / new Number() / new String())
Custom class instances, including subclasses of native classes

Methods are available for deep, shallow, SameValue, SameValueZero, or strict equality comparison. In addition, you can opt into support for circular objects, or performing a "strict" comparison with unconventional property definition, or both. You can also customize any specific type comparison based on your application's use-cases.

By default, npm should resolve the correct build of the package based on your consumption (ESM vs CommonJS). However, if you want to force use of a specific build, they can be located here:

ESM => fast-equals/dist/es/index.mjs
CommonJS => fast-equals/dist/cjs/index.cjs

If you are having any problems, want to request a new feature, or have any questions, please file an issue.

Usage
Available methods
Benchmarks

Usage

import { deepEqual } from 'fast-equals';

console.log(deepEqual({ foo: 'bar' }, { foo: 'bar' })); // true

Available methods

deepEqual

Performs a deep equality comparison on the two objects passed and returns a boolean representing the value equivalency of the objects.

import { deepEqual } from 'fast-equals';

const objectA = { foo: { bar: 'baz' } };
const objectB = { foo: { bar: 'baz' } };

console.log(objectA === objectB); // false
console.log(deepEqual(objectA, objectB)); // true

Comparing `Map`s

Map objects support complex keys (objects, Arrays, etc.), however the spec for key lookups in Map are based on SameZeroValue. If the spec were followed for comparison, the following would always be false:

const mapA = new Map([[{ foo: 'bar' }, { baz: 'quz' }]]);
const mapB = new Map([[{ foo: 'bar' }, { baz: 'quz' }]]);

deepEqual(mapA, mapB);

To support true deep equality of all contents, fast-equals will perform a deep equality comparison for key and value pairs. Therefore, the above would be true.

shallowEqual

Performs a shallow equality comparison on the two objects passed and returns a boolean representing the value equivalency of the objects.

import { shallowEqual } from 'fast-equals';

const nestedObject = { bar: 'baz' };

const objectA = { foo: nestedObject };
const objectB = { foo: nestedObject };
const objectC = { foo: { bar: 'baz' } };

console.log(objectA === objectB); // false
console.log(shallowEqual(objectA, objectB)); // true
console.log(shallowEqual(objectA, objectC)); // false

sameValueEqual

Performs a SameValue comparison on the two objects passed and returns a boolean representing the value equivalency of the objects. In simple terms, this means:

+0 and -0 are not equal
NaN is equal to NaN
All other items are based on referential equality (a === b)

import { sameValueEqual } from 'fast-equals';

const mainObject = { foo: NaN, bar: 'baz' };

const objectA = 'baz';
const objectB = NaN;
const objectC = { foo: NaN, bar: 'baz' };

console.log(sameValueEqual(mainObject.bar, objectA)); // true
console.log(sameValueEqual(mainObject.foo, objectB)); // true
console.log(sameValueEqual(mainObject, objectC)); // false

NOTE: In environments that support Object.is, sameValueEqual is just a re-export of that method.

sameValueZeroEqual

Performs a SameValueZero comparison on the two objects passed and returns a boolean representing the value equivalency of the objects. In simple terms, this means:

+0 and -0 are equal
NaN is equal to NaN
All other items are based on referential equality (a === b)

import { sameValueEqual } from 'fast-equals';

const mainObject = { foo: NaN, bar: 'baz' };

const objectA = 'baz';
const objectB = NaN;
const objectC = { foo: NaN, bar: 'baz' };

console.log(sameValueEqual(mainObject.bar, objectA)); // true
console.log(sameValueEqual(mainObject.foo, objectB)); // true
console.log(sameValueEqual(mainObject, objectC)); // false

strictEqual

Performs a Strict Equality comparison on the two objects passed and returns a boolean representing the referential equality of the objects. In simple terms, this means:

+0 and -0 are equal
NaN is not equal to NaN
All other items are based on referential equality (a === b)

import { strictEqual } from 'fast-equals';

const mainObject = { foo: NaN, bar: 'baz' };

const objectA = 'baz';
const objectB = NaN;
const objectC = { foo: NaN, bar: 'baz' };

console.log(sameValueEqual(mainObject.bar, objectA)); // true
console.log(sameValueEqual(mainObject.foo, objectB)); // false
console.log(sameValueEqual(mainObject, objectC)); // false

NOTE: This is mainly a convenience function, such as needing a default functional equality comparator. Naturally, it is faster to simply compare a === b. :)

circularDeepEqual

Performs the same comparison as deepEqual but supports circular objects. It is slower than deepEqual, so only use if you know circular objects are present.

function Circular(value) {
  this.me = {
    deeply: {
      nested: {
        reference: this,
      },
    },
    value,
  };
}

console.log(circularDeepEqual(new Circular('foo'), new Circular('foo'))); // true
console.log(circularDeepEqual(new Circular('foo'), new Circular('bar'))); // false

Just as with deepEqual, both keys and values are compared for deep equality.

circularShallowEqual

Performs the same comparison as shallowequal but supports circular objects. It is slower than shallowEqual, so only use if you know circular objects are present.

const array = ['foo'];

array.push(array);

console.log(circularShallowEqual(array, ['foo', array])); // true
console.log(circularShallowEqual(array, [array])); // false

strictDeepEqual

Performs the same comparison as deepEqual but performs a strict comparison of the objects. In this includes:

Checking symbol properties
Checking non-enumerable properties in object comparisons
Checking full descriptor of properties on the object to match
Checking non-index properties on arrays
Checking non-key properties on Map / Set objects

const array = [{ foo: 'bar' }];
const otherArray = [{ foo: 'bar' }];

array.bar = 'baz';
otherArray.bar = 'baz';

console.log(strictDeepEqual(array, otherArray)); // true;
console.log(strictDeepEqual(array, [{ foo: 'bar' }])); // false;

strictShallowEqual

Performs the same comparison as shallowEqual but performs a strict comparison of the objects. In this includes:

Checking non-enumerable properties in object comparisons
Checking full descriptor of properties on the object to match
Checking non-index properties on arrays
Checking non-key properties on Map / Set objects

const array = ['foo'];
const otherArray = ['foo'];

array.bar = 'baz';
otherArray.bar = 'baz';

console.log(strictDeepEqual(array, otherArray)); // true;
console.log(strictDeepEqual(array, ['foo'])); // false;

strictCircularDeepEqual

Performs the same comparison as circularDeepEqual but performs a strict comparison of the objects. In this includes:

Checking Symbol properties on the object
Checking non-enumerable properties in object comparisons
Checking full descriptor of properties on the object to match
Checking non-index properties on arrays
Checking non-key properties on Map / Set objects

function Circular(value) {
  this.me = {
    deeply: {
      nested: {
        reference: this,
      },
    },
    value,
  };
}

const first = new Circular('foo');

Object.defineProperty(first, 'bar', {
  enumerable: false,
  value: 'baz',
});

const second = new Circular('foo');

Object.defineProperty(second, 'bar', {
  enumerable: false,
  value: 'baz',
});

console.log(circularDeepEqual(first, second)); // true
console.log(circularDeepEqual(first, new Circular('foo'))); // false

strictCircularShallowEqual

Performs the same comparison as circularShallowEqual but performs a strict comparison of the objects. In this includes:

Checking non-enumerable properties in object comparisons
Checking full descriptor of properties on the object to match
Checking non-index properties on arrays
Checking non-key properties on Map / Set objects

const array = ['foo'];
const otherArray = ['foo'];

array.push(array);
otherArray.push(otherArray);

array.bar = 'baz';
otherArray.bar = 'baz';

console.log(circularShallowEqual(array, otherArray)); // true
console.log(circularShallowEqual(array, ['foo', array])); // false

createCustomEqual

Creates a custom equality comparator that will be used on nested values in the object. Unlike deepEqual and shallowEqual, this is a factory method that receives the default options used internally, and allows you to override the defaults as needed. This is generally for extreme edge-cases, or supporting legacy environments.

The signature is as follows:

interface Cache<Key extends object, Value> {
  delete(key: Key): boolean;
  get(key: Key): Value | undefined;
  set(key: Key, value: any): any;
}

interface ComparatorConfig<Meta> {
  areArrayBuffersEqual: EqualityComparator<Meta>;
  areArraysEqual: EqualityComparator<Meta>;
  areDataViewsEqual: EqualityComparator<Meta>;
  areDatesEqual: EqualityComparator<Meta>;
  areErrorsEqual: EqualityComparator<Meta>;
  areFunctionsEqual: EqualityComparator<Meta>;
  areMapsEqual: EqualityComparator<Meta>;
  areNumbersEqual: EqualityComparator<Meta>;
  areObjectsEqual: EqualityComparator<Meta>;
  arePrimitiveWrappersEqual: EqualityComparator<Meta>;
  areRegExpsEqual: EqualityComparator<Meta>;
  areSetsEqual: EqualityComparator<Meta>;
  areTypedArraysEqual: EqualityComparator<Meta>;
  areUrlsEqual: EqualityComparator<Meta>;
  getUnsupportedCustomComparator: <Type>(a: Type, b: Type, state: State<Meta>, tag: string) => EqualityComparator<Meta>;
}

function createCustomEqual<Meta>(options: {
  circular?: boolean;
  createCustomConfig?: (defaultConfig: ComparatorConfig<Meta>) => Partial<ComparatorConfig<Meta>>;
  createInternalComparator?: (
    compare: <A, B>(a: A, b: B, state: State<Meta>) => boolean,
  ) => (a: any, b: any, indexOrKeyA: any, indexOrKeyB: any, parentA: any, parentB: any, state: State<Meta>) => boolean;
  createState?: () => { cache?: Cache; meta?: Meta };
  strict?: boolean;
}): <A, B>(a: A, b: B) => boolean;

Create a custom equality comparator. This allows complete control over building a bespoke equality method, in case your use-case requires a higher degree of performance, legacy environment support, or any other non-standard usage. The recipes provide examples of use in different use-cases, but if you have a specific goal in mind and would like assistance feel free to file an issue.

NOTE: Map implementations compare equality for both keys and value. When using a custom comparator and comparing equality of the keys, the iteration index is provided as both indexOrKeyA and indexOrKeyB to help use-cases where ordering of keys matters to equality.

getUnsupportedCustomComparator

If you want to compare objects that have a custom @@toStringTag, you can provide a map of the custom tags you want to support via the getUnsupportedCustomComparator option. See this recipe for an example.

Recipes

Some recipes have been created to provide examples of use-cases for createCustomEqual. Even if not directly applicable to the problem you are solving, they can offer guidance of how to structure your solution.

Benchmarks

All benchmarks were performed on an i9-11900H Ubuntu Linux 24.04 laptop with 64GB of memory using NodeJS version 24.11.1, and are based on averages of running comparisons based deep equality on the following object types:

Primitives (String, Number, null, undefined)
Function
Object
Array
Date
RegExp
react elements
A mixed object with a combination of all the above types

┌────────────────────────────────────────┬────────────────┐
│ Name                                   │ Ops / sec      │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (passed)                   │ 1544237.29413  │
├────────────────────────────────────────┼────────────────┤
│ fast-deep-equal (passed)               │ 1328583.767745 │
├────────────────────────────────────────┼────────────────┤
│ react-fast-compare (passed)            │ 1301727.296375 │
├────────────────────────────────────────┼────────────────┤
│ shallow-equal-fuzzy (passed)           │ 1225981.400919 │
├────────────────────────────────────────┼────────────────┤
│ nano-equal (failed)                    │ 969495.538753  │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (circular) (passed)        │ 813716.49516   │
├────────────────────────────────────────┼────────────────┤
│ dequal/lite (passed)                   │ 780805.627339  │
├────────────────────────────────────────┼────────────────┤
│ dequal (passed)                        │ 767208.995048  │
├────────────────────────────────────────┼────────────────┤
│ underscore.isEqual (passed)            │ 490695.830468  │
├────────────────────────────────────────┼────────────────┤
│ assert.deepStrictEqual (passed)        │ 471011.425391  │
├────────────────────────────────────────┼────────────────┤
│ lodash.isEqual (passed)                │ 296064.057382  │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (strict) (passed)          │ 225894.800964  │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (strict circular) (passed) │ 195657.732354  │
├────────────────────────────────────────┼────────────────┤
│ deep-eql (passed)                      │ 162718.102328  │
├────────────────────────────────────────┼────────────────┤
│ deep-equal (passed)                    │ 954.172311     │
└────────────────────────────────────────┴────────────────┘

Testing mixed objects not equal...
┌────────────────────────────────────────┬────────────────┐
│ Name                                   │ Ops / sec      │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (passed)                   │ 5112341.000979 │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (circular) (passed)        │ 3501225.300307 │
├────────────────────────────────────────┼────────────────┤
│ fast-deep-equal (passed)               │ 3471838.735181 │
├────────────────────────────────────────┼────────────────┤
│ react-fast-compare (passed)            │ 3439612.908273 │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (strict) (passed)          │ 1797319.423491 │
├────────────────────────────────────────┼────────────────┤
│ fast-equals (strict circular) (passed) │ 1534168.229167 │
├────────────────────────────────────────┼────────────────┤
│ dequal/lite (passed)                   │ 1357981.758571 │
├────────────────────────────────────────┼────────────────┤
│ dequal (passed)                        │ 1328078.173967 │
├────────────────────────────────────────┼────────────────┤
│ shallow-equal-fuzzy (failed)           │ 1224747.272118 │
├────────────────────────────────────────┼────────────────┤
│ nano-equal (passed)                    │ 1087373.99615  │
├────────────────────────────────────────┼────────────────┤
│ underscore.isEqual (passed)            │ 927298.592729  │
├────────────────────────────────────────┼────────────────┤
│ lodash.isEqual (passed)                │ 387294.235476  │
├────────────────────────────────────────┼────────────────┤
│ deep-eql (passed)                      │ 186028.168827  │
├────────────────────────────────────────┼────────────────┤
│ assert.deepStrictEqual (passed)        │ 21261.312424   │
├────────────────────────────────────────┼────────────────┤
│ deep-equal (passed)                    │ 3782.329948    │
└────────────────────────────────────────┴────────────────┘

Caveats that impact the benchmark (and accuracy of comparison):

Maps, Promises, and Sets were excluded from the benchmark entirely because no library other than deep-eql fully supported their comparison
fast-deep-equal, react-fast-compare and nano-equal throw on objects with null as prototype (Object.create(null))
assert.deepStrictEqual does not support NaN or SameValue equality for dates
deep-eql does not support SameValue equality for zero equality (positive and negative zero are not equal)
deep-equal does not support NaN and does not strictly compare object type, or date / regexp values, nor uses SameValue equality for dates
fast-deep-equal does not support NaN or SameValue equality for dates
nano-equal does not strictly compare object property structure, array length, or object type, nor SameValue equality for dates
react-fast-compare does not support NaN or SameValue equality for dates, and does not compare function equality
shallow-equal-fuzzy does not strictly compare object type or regexp values, nor SameValue equality for dates
underscore.isEqual does not support SameValue equality for primitives or dates

All of these have the potential of inflating the respective library's numbers in comparison to fast-equals, but it was the closest apples-to-apples comparison I could create of a reasonable sample size. It should be noted that react elements can be circular objects, however simple elements are not; I kept the react comparison very basic to allow it to be included.

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