fix: only use instances singleton within the browser

Author: blittle

src/HelmetData.js

@@ -7,26 +7,29 @@ export function clearInstances() {
 }
 
 export default class HelmetData {
+  instances = [];
+
   value = {
     setHelmet: serverState => {
       this.context.helmet = serverState;
     },
     helmetInstances: {
-      get: () => instances,
+      get: () => (this.canUseDOM ? instances : this.instances),
       add: instance => {
-        instances.push(instance);
+        (this.canUseDOM ? instances : this.instances).push(instance);
       },
       remove: instance => {
-        const index = instances.indexOf(instance);
-        instances.splice(index, 1);
+        const index = (this.canUseDOM ? instances : this.instances).indexOf(instance);
+        (this.canUseDOM ? instances : this.instances).splice(index, 1);
       },
     },
   };
 
-  constructor(context) {
+  constructor(context, canUseDOM = typeof document !== 'undefined') {
     this.context = context;
+    this.canUseDOM = canUseDOM;
 
-    if (!HelmetData.canUseDOM) {
+    if (!canUseDOM) {
       context.helmet = mapStateOnServer({
         baseTag: [],
         bodyAttributes: {},

src/Provider.js

@@ -36,7 +36,7 @@ export default class Provider extends Component {
   constructor(props) {
     super(props);
 
-    this.helmetData = new HelmetData(this.props.context);
+    this.helmetData = new HelmetData(this.props.context, Provider.canUseDOM);
   }
 
   render() {