Automatically use <base href> in useBasename

Fixes #94

CHANGES.md

@@ -1,8 +1,11 @@
 ## HEAD
 
 - `useBasename` transparently handles trailing slashes (see [#108])
+- `useBasename` automatically uses the value of <base href> when no
+  `basename` option is provided (see [#94])
 
 [#108]: https://github.com/rackt/history/pull/108
+[#94]: https://github.com/rackt/history/issues/94
 
 ## [v1.12.6]
 > Oct 25, 2015

docs/BasenameSupport.md

@@ -23,3 +23,7 @@ Basename-enhanced histories also automatically prepend the basename to paths use
 history.createPath('/the/path') // /base/the/path
 history.pushState(null, '/the/path') // push /base/the/path
 ```
+
+### Using <base href>
+
+In HTML documents, you can use the [`<base>` element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base)'s `href` attribute to specify a `basename` for the page. This way, you don't have to manually pass the `basename` property to your `createHistory` function.

modules/useBasename.js

@@ -1,3 +1,4 @@
+import { canUseDOM } from './ExecutionEnvironment'
 import runTransitionHook from './runTransitionHook'
 import parsePath from './parsePath'
 
@@ -6,6 +7,15 @@ function useBasename(createHistory) {
     let { basename, ...historyOptions } = options
     let history = createHistory(historyOptions)
 
+    // Automatically use the value of <base href> in HTML
+    // documents as basename if it's not explicitly given.
+    if (basename == null && canUseDOM) {
+      let base = document.getElementsByTagName('base')[0]
+
+      if (base)
+        basename = base.href
+    }
+
     function addBasename(location) {
       if (basename && location.basename == null) {
         if (location.pathname.indexOf(basename) === 0) {