Improve the is.observable check

Fixes #72

package.json

@@ -43,9 +43,6 @@
 		"verify",
 		"compare"
 	],
-	"dependencies": {
-		"symbol-observable": "^1.2.0"
-	},
 	"devDependencies": {
 		"@sindresorhus/tsconfig": "^0.1.0",
 		"@types/jsdom": "^11.12.0",

source/index.ts

@@ -2,7 +2,6 @@
 /// <reference lib="es2017.sharedmemory"/>
 /// <reference lib="esnext.asynciterable"/>
 /// <reference lib="dom"/>
-import symbolObservable from 'symbol-observable';
 
 type TypedArray = Int8Array | Uint8Array | Uint8ClampedArray | Int16Array | Uint16Array | Int32Array | Uint32Array | Float32Array | Float64Array;
 type Primitive = null | undefined | string | number | boolean | Symbol;
@@ -264,7 +263,22 @@ namespace is { // tslint:disable-line:no-namespace
 	export const domElement = (value: unknown): value is DomElement => object(value) && (value as DomElement).nodeType === NODE_TYPE_ELEMENT && string((value as DomElement).nodeName) &&
 		!plainObject(value) && DOM_PROPERTIES_TO_CHECK.every(property => property in (value as DomElement));
 
-	export const observable = (value: unknown) => Boolean(value && (value as any)[symbolObservable] && value === (value as any)[symbolObservable]());
+	export const observable = (value: unknown) => {
+		if (!value) {
+			return false;
+		}
+
+		if ((value as any)[Symbol.observable] && value === (value as any)[Symbol.observable]()) {
+			return true;
+		}
+
+		if ((value as any)['@@observable'] && value === (value as any)['@@observable']()) {
+			return true;
+		}
+
+		return false;
+	};
+
 	export const nodeStream = (value: unknown): value is NodeStream => !nullOrUndefined(value) && isObject(value) as unknown && function_((value as NodeStream).pipe) && !observable(value);
 
 	export const infinite = (value: unknown) => value === Infinity || value === -Infinity;

source/tests/test.ts

@@ -345,6 +345,14 @@ const types = new Map<string, Test>([
 			document.createDocumentFragment()
 		]
 	}],
+	['observable', {
+		is: is.observable,
+		fixtures: [
+			new Observable(),
+			new Subject(),
+			new ZenObservable(() => {}) // tslint:disable-line:no-empty
+		]
+	}],
 	['nodeStream', {
 		is: is.nodeStream,
 		fixtures: [
@@ -359,14 +367,6 @@ const types = new Map<string, Test>([
 			new Stream.Writable()
 		]
 	}],
-	['observable', {
-		is: is.observable,
-		fixtures: [
-			new Observable(),
-			new Subject(),
-			new ZenObservable(() => {})	// tslint:disable-line:no-empty
-		]
-	}],
 	['infinite', {
 		is: is.infinite,
 		fixtures: [
@@ -751,6 +751,10 @@ test('is.domElement', t => {
 	t.false(is.domElement({nodeType: 1, nodeName: 'div'}));
 });
 
+test('is.observable', t => {
+	testType(t, 'observable');
+});
+
 test('is.nodeStream', t => {
 	testType(t, 'nodeStream');
 });