Improve the robustness and strictness of the checks

index.js

@@ -6,12 +6,13 @@ module.exports = value => {
 	}
 
 	// eslint-disable-next-line no-use-extend-native/no-use-extend-native
-	if (value[Symbol.observable] && value === value[Symbol.observable]()) {
-		return true;
+	if (typeof Symbol.observable === 'symbol' && typeof value[Symbol.observable] === 'function') {
+		// eslint-disable-next-line no-use-extend-native/no-use-extend-native
+		return value === value[Symbol.observable]();
 	}
 
-	if (value['@@observable'] && value === value['@@observable']()) {
-		return true;
+	if (typeof value['@@observable'] === 'function') {
+		return value === value['@@observable']();
 	}
 
 	return false;

test.js → test/main.js

@@ -4,7 +4,7 @@ import {of as rxOf} from 'rxjs';
 import * as most from 'most';
 import {Stream as xstream} from 'xstream';
 import {IndefiniteObservable} from 'indefinite-observable';
-import isObservable from '.';
+import isObservable from '..';
 
 test('main', t => {
 	t.true(isObservable(zenObservable.of(1)));

test/strict.js

@@ -0,0 +1,20 @@
+import test from 'ava';
+import isObservable from '..';
+
+const fake1 = {'@@observable': true};
+const fake2 = {undefined: true};
+const fake3 = {
+	undefined() {
+		return this;
+	}
+};
+
+test('strict', t => {
+	// Symbol.observable is defined via AVA
+	delete Symbol.observable;
+	// eslint-disable-next-line no-use-extend-native/no-use-extend-native
+	t.is(Symbol.observable, undefined);
+	t.false(isObservable(fake1));
+	t.false(isObservable(fake2));
+	t.false(isObservable(fake3));
+});