test presence of IntersectionObserver constructor before adding hook

lib/hooks/hooks-frame.js

@@ -68,61 +68,63 @@ this.hooksFrame = this.hooksFrame || (() => {
 			}
 		}));
 
-		const LOAD_OBSERVED_ELEMENTS_EVENT = "single-file-load-observed-elements";
-
-		const IntersectionObserver = window.IntersectionObserver;
-		const observeIntersection = IntersectionObserver.prototype.observe;
-		const unobserveIntersection = IntersectionObserver.prototype.unobserve;
-		const observedElements = new Map();
-		const observers = new Map();
-		let warningIntersectionObserverDisplayed;
-		window.__defineGetter__("IntersectionObserver", () => new Proxy(IntersectionObserver, {
-			construct(IntersectionObserver, argumentsList) {
-				if (!warningIntersectionObserverDisplayed) {
-					console.warn("SingleFile is hooking the IntersectionObserver API to detect and load deferred images."); // eslint-disable-line no-console
-				}
-				const intersectionObserver = new IntersectionObserver(...argumentsList);
-				const callback = argumentsList[0];
-				const options = argumentsList[1];
-				intersectionObserver.observe = function (targetElement) {
-					let targetElements = observedElements.get(intersectionObserver);
-					if (!targetElements) {
-						targetElements = [];
-						observedElements.set(intersectionObserver, targetElements);
+		if (window.IntersectionObserver) {
+			const LOAD_OBSERVED_ELEMENTS_EVENT = "single-file-load-observed-elements";
+		
+			const IntersectionObserver = window.IntersectionObserver;
+			const observeIntersection = IntersectionObserver.prototype.observe;
+			const unobserveIntersection = IntersectionObserver.prototype.unobserve;
+			const observedElements = new Map();
+			const observers = new Map();
+			let warningIntersectionObserverDisplayed;
+			window.__defineGetter__("IntersectionObserver", () => new Proxy(IntersectionObserver, {
+				construct(IntersectionObserver, argumentsList) {
+					if (!warningIntersectionObserverDisplayed) {
+						console.warn("SingleFile is hooking the IntersectionObserver API to detect and load deferred images."); // eslint-disable-line no-console
 					}
-					targetElements.push(targetElement);
-					return observeIntersection.call(intersectionObserver, targetElement);
-				};
-				intersectionObserver.unobserve = function (targetElement) {
-					let targetElements = observedElements.get(intersectionObserver);
-					if (targetElements) {
-						targetElements = targetElements.filter(element => element <= targetElement);
-						if (targetElements.length) {
+					const intersectionObserver = new IntersectionObserver(...argumentsList);
+					const callback = argumentsList[0];
+					const options = argumentsList[1];
+					intersectionObserver.observe = function (targetElement) {
+						let targetElements = observedElements.get(intersectionObserver);
+						if (!targetElements) {
+							targetElements = [];
 							observedElements.set(intersectionObserver, targetElements);
-						} else {
-							observedElements.delete(intersectionObserver);
 						}
-					}
-					return unobserveIntersection.call(intersectionObserver, targetElement);
-				};
-				observers.set(intersectionObserver, { callback, options });
-				return intersectionObserver;
-			}
-		}));
-		window.__defineSetter__("IntersectionObserver", () => { });
-		addEventListener(LOAD_OBSERVED_ELEMENTS_EVENT, () => {
-			Array.from(observers).forEach(([intersectionObserver, observer]) => {
-				observer.callback(observedElements.get(intersectionObserver).map(target => {
-					const boundingClientRect = target.getBoundingClientRect();
-					const intersectionRect = target.getBoundingClientRect();
-					const isIntersecting = true;
-					const intersectionRatio = 1;
-					const rootBounds = observer.options && observer.options.root ? observer.options.root.getBoundingClientRect() : document.documentElement.getBoundingClientRect();
-					const time = 0;
-					return { target, intersectionRatio, boundingClientRect, intersectionRect, isIntersecting, rootBounds, time };
-				}), intersectionObserver);
-			});
-		}, false);
+						targetElements.push(targetElement);
+						return observeIntersection.call(intersectionObserver, targetElement);
+					};
+					intersectionObserver.unobserve = function (targetElement) {
+						let targetElements = observedElements.get(intersectionObserver);
+						if (targetElements) {
+							targetElements = targetElements.filter(element => element <= targetElement);
+							if (targetElements.length) {
+								observedElements.set(intersectionObserver, targetElements);
+							} else {
+								observedElements.delete(intersectionObserver);
+							}
+						}
+						return unobserveIntersection.call(intersectionObserver, targetElement);
+					};
+					observers.set(intersectionObserver, { callback, options });
+					return intersectionObserver;
+				}
+			}));
+			window.__defineSetter__("IntersectionObserver", () => { });
+			addEventListener(LOAD_OBSERVED_ELEMENTS_EVENT, () => {
+				Array.from(observers).forEach(([intersectionObserver, observer]) => {
+					observer.callback(observedElements.get(intersectionObserver).map(target => {
+						const boundingClientRect = target.getBoundingClientRect();
+						const intersectionRect = target.getBoundingClientRect();
+						const isIntersecting = true;
+						const intersectionRatio = 1;
+						const rootBounds = observer.options && observer.options.root ? observer.options.root.getBoundingClientRect() : document.documentElement.getBoundingClientRect();
+						const time = 0;
+						return { target, intersectionRatio, boundingClientRect, intersectionRect, isIntersecting, rootBounds, time };
+					}), intersectionObserver);
+				});
+			}, false);
+		}
 	}
 
 })();