mirror: A fast canonicalisation utility for stable object equality
feat: objects with toJSON uses that for its identity
Objects with a `toJSON` method (URL, Temporal, custom types) now serialize
as whatever it returns.
+42B, perf-neutral on realistic shapes.
Marais Rossouw
(Jun 25, 2026, 3:42 PM +1000)
0ff606e7
5e8d86a4
+40
-3
+34
-2
lib/mod.test.ts
+34
-2
lib/mod.test.ts
···
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assertEquals(identify(() => {}), identify(undefined));
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});
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// ~> toJSON
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+
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Deno.test('toJSON :: result is serialized in place', () => {
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assertEquals(identify({ toJSON: () => ({ a: 1, b: [2] }) }), identify({ a: 1, b: [2] }));
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assertEquals(identify({ ignored: 99, toJSON: () => ({ a: 1 }) }), identify({ a: 1 }));
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// Primitive and undefined results behave like that primitive / a drop.
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assertEquals(identify({ toJSON: () => 42 }), identify(42));
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assertEquals(identify({ toJSON: () => 'x' }), identify('x'));
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assertEquals(identify({ a: 1, b: { toJSON: () => undefined } }), identify({ a: 1 }));
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});
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Deno.test('toJSON :: URL serializes as its href', () => {
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assertEquals(
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identify(new URL('https://example.com/a?b=1#c')),
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identify('https://example.com/a?b=1#c'),
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);
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assertEquals(identify(new URL('https://a.test/')), identify(new URL('https://a.test/')));
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assertNotEquals(identify(new URL('https://a.test/')), identify(new URL('https://b.test/')));
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});
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Deno.test('toJSON :: builtins with special forms ignore toJSON', () => {
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assertEquals(identify(new Date(0)), 'd0');
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assertNotEquals(identify(new Date(0)), identify(new Date(0).toJSON()));
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assertEquals(identify(Buffer.from([1, 2])), identify(new Uint8Array([1, 2])));
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});
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Deno.test('toJSON :: only applied once, like JSON.stringify', () => {
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const o: any = { a: 1 };
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o.toJSON = () => o;
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assertEquals(identify(o), identify({ a: 1 }));
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});
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// ~> Dates & RegExps
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Deno.test('dates :: equal instants match, different instants differ', () => {
···
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assertWire('uint8 clamped array', new Uint8ClampedArray([1, 255]), 'o0n11n255');
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assertWire('uint16 array', new Uint16Array([1, 2]), 'o0n11n2');
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assertWire('uint32 array', new Uint32Array([1, 2]), 'o0n11n2');
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//assertWire('custom toJSON', { toJSON: () => ({ a: 1, b: [2] }) }, 'oan1ban2');
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//assertWire('url', new URL('https://example.com/a?b=1#c'), 'shttps://example.com/a?b=1#c');
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assertWire('custom toJSON', { toJSON: () => ({ a: 1, b: [2] }) }, 'oan1ban2');
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assertWire('url', new URL('https://example.com/a?b=1#c'), 'shttps://example.com/a?b=1#c');
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});
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Deno.test('format :: circular array back-reference', () => {
+5
lib/mod.ts
+5
lib/mod.ts
···
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if (!is_arr) {
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if (input instanceof Date) return 'd' + +input;
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if (input instanceof RegExp) return 'r' + input.source + input.flags;
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if (typeof input.toJSON === 'function' && !ArrayBuffer.isView(input)) {
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input = input.toJSON();
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if (typeof input !== 'object' || input === null) return walk(input, seen);
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is_arr = Array.isArray(input);
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}
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}
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let ref: any = seen.indexOf(input);
+1
-1
readme.md
+1
-1
readme.md
···
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- 🧬 **Canonical.** The same shape always produces the same id.
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- 🌀 **Deep and cycle-safe.** Handles nested objects, arrays, sets, maps, and circular references.
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- 🏎 **Fast.** See the [benchmarks](#-benchmark).
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- 🪶 **Tiny.** Around 529B minified and gzipped, with zero
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- 🪶 **Tiny.** Around 571B minified and gzipped, with zero
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[dependencies](https://npm.anvaka.com/#/view/2d/object-identity/).
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## ⚙️ Install