Improve performance

benchmark.js

@@ -0,0 +1,57 @@
+import {inspect} from 'node:util';
+import {runInNewContext} from 'node:vm';
+import isPlainObject from 'is-plain-obj';
+
+const runBenchmarks = () => {
+	for (const value of values) {
+		const name = value instanceof Error ? String(Error) : inspect(value);
+		const paddedName = name.padEnd(50);
+		console.time(paddedName);
+		runLoop(value);
+		console.timeEnd(paddedName);
+	}
+};
+
+const runLoop = value => {
+	for (let index = 0; index < 1e8; index += 1) {
+		isPlainObject(value);
+	}
+};
+
+const values = [
+	undefined,
+	0,
+	0n,
+	'',
+	true,
+	Symbol(''),
+	() => {},
+	// eslint-disable-next-line func-names
+	(function namedFunc() {}),
+	null,
+	{},
+	Math,
+	new Set([]),
+	new ArrayBuffer(0),
+	Promise.resolve(),
+	Object.create(null),
+	new Intl.Locale('en'),
+	// eslint-disable-next-line no-new-object
+	new Object({prop: true}),
+	new class Class {}(),
+	[],
+	/regexp/,
+	new Error('test'),
+	new Date(),
+	(function () {
+		// eslint-disable-next-line prefer-rest-params
+		return arguments;
+	})(),
+	new Proxy({}, {})
+];
+
+// Warm up V8 optimization.
+// Must go through every branch of the code, which requires using an object from a different realm.
+runLoop(runInNewContext('({})'));
+
+runBenchmarks();

index.js

@@ -1,8 +1,8 @@
 export default function isPlainObject(value) {
-	if (Object.prototype.toString.call(value) !== '[object Object]') {
+	if (typeof value !== 'object' || value === null) {
 		return false;
 	}
 
 	const prototype = Object.getPrototypeOf(value);
-	return prototype === null || Object.getPrototypeOf(prototype) === null;
+	return (prototype === null || prototype === Object.prototype || Object.getPrototypeOf(prototype) === null) && !(Symbol.toStringTag in value) && !(Symbol.iterator in value);
 }