bug/issue 147 re-use existing DOM shim globals if they exist

src/dom-shim.js

@@ -9,7 +9,7 @@
 
 // https://developer.mozilla.org/en-US/docs/Web/API/Node
 // EventTarget <- Node
 // TODO should be an interface?
 class Node extends EventTarget {
   // eslint-disable-next-line
   cloneNode(deep) {
@@ -104,7 +104,7 @@
     this.content = new DocumentFragment();
   }
 
   // TODO open vs closed shadow root
   set innerHTML(html) {
     if (this.content) {
       this.content.innerHTML = `
@@ -137,7 +137,8 @@
 
 // mock top level aliases (globalThis === window)
 // https://developer.mozilla.org/en-US/docs/Web/API/Window
-globalThis.addEventListener = noop;
-globalThis.document = new Document();
-globalThis.customElements = new CustomElementsRegistry();
-globalThis.HTMLElement = HTMLElement;
+// make this "idempotent" for now until a better idea comes along - https://github.com/ProjectEvergreen/wcc/discussions/145
+globalThis.addEventListener = globalThis.addEventListener ?? noop;
+globalThis.document = globalThis.document ?? new Document();
+globalThis.customElements = globalThis.customElements ?? new CustomElementsRegistry();
+globalThis.HTMLElement = globalThis.HTMLElement ?? HTMLElement;