feat(index, prefetch) Add support for higher prio fetches

demo/index.html

@@ -20,6 +20,7 @@
     <script src="../dist/quicklink.umd.js"></script>
     <script>
         quicklink();
+        // quicklink({ priority: 'high' });
     </script>
     <script type="module">
     // import quicklink from "../dist/quicklink.mjs";

src/index.mjs

@@ -53,9 +53,10 @@ function extractInViewportLinks(el) {
  * @param {Object} options - Configuration options for quicklink
  * @param {Array} options.urls - Array of URLs to prefetch (override)
  * @param {Object} options.el - DOM element to prefetch in-viewport links of
+ * @param {string} options.priority - Attempt to fetch with higher priority (low or high)
  */
 export default function (options) {
-  options = options || {};
+  options = options || { priority: 'low' };
   requestIdleCallback(() => {
     if ('connection' in navigator) {
       // Don't prefetch if the user is on 2G..
@@ -69,12 +70,12 @@ export default function (options) {
     }
     // Prefetch an array of URLs if supplied (as an override)
     if (options.urls !== undefined && options.urls.length > 0) {
-      prefetchLinks(options.urls);
+      prefetchLinks(options.urls, options.priority);
     } else {
       // Element to extract in-viewport links for
       const el = options.el || document;
       extractInViewportLinks(el).then(urls => {
-        prefetchLinks(urls);
+        prefetchLinks(urls, options.priority);
       });
     }
   });

src/prefetch.mjs

@@ -1,3 +1,5 @@
+// Inspired by the prefetching logic in Gatsby.js
+
 const support = function (feature) {
   if (typeof document === `undefined`) {
     return false;
@@ -11,23 +13,66 @@ const support = function (feature) {
     return false;
   }
 };
+
 const linkPrefetchStrategy = function (url) {
-  if (typeof document === `undefined`) {
-    return;
-  }
-  const link = document.createElement(`link`);
-  link.setAttribute(`rel`, `prefetch`);
-  link.setAttribute(`href`, url);
-  const parentElement =
-    document.getElementsByTagName(`head`)[0] ||
-    document.getElementsByName(`script`)[0].parentNode;
-  parentElement.appendChild(link);
+  return new Promise((resolve, reject) => {
+    if (typeof document === `undefined`) {
+      reject();
+      return;
+    }
+
+    const link = document.createElement(`link`);
+    link.setAttribute(`rel`, `prefetch`);
+    link.setAttribute(`href`, url);
+
+    link.onload = resolve;
+    link.onerror = reject;
+
+    const parentElement =
+      document.getElementsByTagName(`head`)[0] ||
+      document.getElementsByName(`script`)[0].parentNode;
+    parentElement.appendChild(link);
+  });
 };
+
 const xhrPrefetchStrategy = function (url) {
-  const req = new XMLHttpRequest();
-  req.open(`GET`, url, true);
-  req.withCredentials = true;
-  req.send(null);
+  return new Promise((resolve, reject) => {
+    const req = new XMLHttpRequest();
+    req.open(`GET`, url, true);
+    req.withCredentials = true;
+
+    req.onload = () => {
+      if (req.status === 200) {
+        resolve();
+      } else {
+        reject();
+      }
+    };
+
+    req.send(null);
+  });
+};
+
+const highPriFetchStrategy = function (url) {
+  return new Promise((resolve, reject) => {
+    // TODO: Investigate using preload for high-priority
+    // fetches. May have to sniff file-extension to provide
+    // valid 'as' values. In the future, we may be able to
+    // use Priority Hints here.
+    if (self.fetch === undefined) {
+      xhrPrefetchStrategy(url)
+          .then(() => {
+            resolve();
+          });
+    } else {
+      // As of 2018, fetch() is high-priority in Chrome
+      // and medium-priority in Safari.
+      fetch(url, {credentials: `include`})
+          .then(() => {
+            resolve();
+          });
+    }
+  });
 };
 
 const supportedPrefetchStrategy = support(`prefetch`)
@@ -36,22 +81,35 @@ const supportedPrefetchStrategy = support(`prefetch`)
 
 const preFetched = {};
 
-const prefetch = function (url) {
-  if (preFetched[url]) {
-    return;
-  }
-  preFetched[url] = true;
-  supportedPrefetchStrategy(url);
+const prefetch = function (url, priority) {
+  return new Promise(resolve => {
+    if (preFetched[url]) {
+      resolve();
+      return;
+    }
+
+    if (priority && priority === `high`) {
+      highPriFetchStrategy(url);
+    } else {
+      supportedPrefetchStrategy(url)
+          .then(() => {
+            resolve();
+            preFetched[url] = true;
+          })
+          .catch(() => { });
+    };
+  });
 };
 
 /**
  * Prefetch an array of URLs using rel=prefetch
  * if supported. Falls back to XHR otherwise.
- * @param {Array} urls Array of URLs to prefetch
+ * @param {Array} urls - Array of URLs to prefetch
+ * @param {string} priority - "priority" of the request
  */
-const prefetchLinks = function (urls) {
+const prefetchLinks = function (urls, priority) {
   urls.forEach(url => {
-    prefetch(url);
+    prefetch(url, priority);
   });
 };