Implement option for setting cookies in highest available domain.

One could also say lowest, I guess.

src/cookies.js

@@ -54,10 +54,40 @@ export function getCookie(win, name) {
  * @param {string} name
  * @param {string} value
  * @param {time} expirationTime
+ * @param {{highestAvailableDomain:boolean}=} opt_options
+ *     - highestAvailableDomain: If true, set the cookie at the widest domain
+ *       scope allowed by the browser. E.g. on example.com if we are currently
+ *       on www.example.com.
  */
-export function setCookie(win, name, value, expirationTime) {
+export function setCookie(win, name, value, expirationTime, opt_options) {
+  if (opt_options && opt_options.highestAvailableDomain) {
+    const parts = win.location.hostname.split('.');
+    let domain = parts[parts.length - 1];
+    for (let i = parts.length - 2; i >= 0; i--) {
+      domain = parts[i] + '.' + domain;
+      trySetCookie(win, name, value, expirationTime, domain);
+      if (getCookie(win, name) == value) {
+        return;
+      }
+    }
+  } else {
+    trySetCookie(win, name, value, expirationTime, undefined);
+  }
+}
+
+/**
+ * Attempt to set a cookie with the given params.
+ *
+ * @param {!Window} win
+ * @param {string} name
+ * @param {string} value
+ * @param {time} expirationTime
+ * @param {string|undefined} domain
+ */
+function trySetCookie(win, name, value, expirationTime, domain) {
   win.document.cookie = encodeURIComponent(name) + '=' +
       encodeURIComponent(value) +
       '; path=/' +
+      (domain ? '; domain=' + domain : '') +
       '; expires=' + new Date(expirationTime).toUTCString();
 }

test/functional/test-cookies.js

@@ -56,4 +56,38 @@ describe('getCookie', () => {
     expect(doc.cookie).to.equal(
         'c%261=v%261; path=/; expires=Fri, 13 Nov 2015 02:52:39 GMT');
   });
+
+  it('should write the cookie to the right domain', () => {
+    function test(hostname, targetDomain, opt_noset) {
+      let cookie;
+      const doc = {
+        set cookie(val) {
+          if (val.indexOf('; domain=' + targetDomain) != -1) {
+            cookie = val;
+          }
+        },
+        get cookie() {
+          return cookie;
+        }
+      };
+      setCookie({document: doc, location: {hostname: hostname}},
+          'c&1', 'v&1', 1447383159853, {
+            highestAvailableDomain: true
+          });
+      if (opt_noset) {
+        expect(cookie).to.be.undefined;
+      } else {
+        expect(cookie).to.equal(
+           'c%261=v%261; path=/; domain=' + targetDomain +
+            '; expires=Fri, 13 Nov 2015 02:52:39 GMT');
+      }
+    }
+    test('www.example.com', 'example.com');
+    test('123.www.example.com', 'example.com');
+    test('example.com', 'example.com');
+    test('www.example.com', 'www.example.com');
+    test('123.www.example.com', '123.www.example.com');
+    test('www.example.net', 'example.com', true);
+    test('example.net', 'example.com', true);
+  });
 });