[added] Router.History

[changed] goBack returns a boolean value

Users can use Router.History.length to determine if the router has
any history. router.goBack() is a no-op when the router does not
have any history and emits a warning.

Fixes remix-run#408

modules/index.js

@@ -17,3 +17,5 @@ exports.State = require('./mixins/State');
 
 exports.create = require('./utils/createRouter');
 exports.run = require('./utils/runRouter');
+
+exports.History = require('./utils/History');

modules/locations/HashLocation.js

@@ -1,4 +1,5 @@
 var LocationActions = require('../actions/LocationActions');
+var History = require('../utils/History');
 var Path = require('../utils/Path');
 
 /**
@@ -29,6 +30,9 @@ function ensureSlash() {
 var _changeListeners = [];
 
 function notifyChange(type) {
+  if (type === LocationActions.PUSH)
+    History.length += 1;
+
   var change = {
     path: getHashPath(),
     type: type
@@ -87,7 +91,7 @@ var HashLocation = {
 
   pop: function () {
     _actionType = LocationActions.POP;
-    window.history.back();
+    History.back();
   },
 
   getCurrentPath: getHashPath,

modules/locations/HistoryLocation.js

@@ -1,4 +1,5 @@
 var LocationActions = require('../actions/LocationActions');
+var History = require('../utils/History');
 var Path = require('../utils/Path');
 
 /**
@@ -51,6 +52,7 @@ var HistoryLocation = {
 
   push: function (path) {
     window.history.pushState({ path: path }, '', Path.encode(path));
+    History.length += 1;
     notifyChange(LocationActions.PUSH);
   },
 
@@ -60,7 +62,7 @@ var HistoryLocation = {
   },
 
   pop: function () {
-    window.history.back();
+    History.back();
   },
 
   getCurrentPath: getWindowPath,

modules/locations/RefreshLocation.js

@@ -17,6 +17,8 @@ var RefreshLocation = {
   },
 
   pop: function () {
+    // History will always have length 1 when using full
+    // page refreshes, so use window.history directly.
     window.history.back();
   },
 

modules/locations/TestLocation.js

@@ -1,4 +1,6 @@
+var invariant = require('react/lib/invariant');
 var LocationActions = require('../actions/LocationActions');
+var History = require('../utils/History');
 
 var _listener;
 
@@ -7,6 +9,10 @@ function notifyChange(type) {
     _listener({ path: TestLocation.getCurrentPath(), type: type });
 }
 
+function updateHistoryLength() {
+  History.length = TestLocation.history.length;
+}
+
 /**
  * A location that is convenient for testing and does not
  * require a DOM. You should manually setup TestLocation.history
@@ -19,20 +25,28 @@ var TestLocation = {
   addChangeListener: function (listener) {
     // TestLocation only ever supports a single listener at a time.
     _listener = listener;
+    updateHistoryLength();
   },
 
   push: function (path) {
     TestLocation.history.push(path);
+    updateHistoryLength();
     notifyChange(LocationActions.PUSH);
   },
 
   replace: function (path) {
+    invariant(
+      History.length,
+      'You cannot replace the current path with no history'
+    );
+
     TestLocation.history[TestLocation.history.length - 1] = path;
     notifyChange(LocationActions.REPLACE);
   },
 
   pop: function () {
     TestLocation.history.pop();
+    updateHistoryLength();
     notifyChange(LocationActions.POP);
   },
 

modules/utils/History.js

@@ -0,0 +1,29 @@
+var invariant = require('react/lib/invariant');
+var canUseDOM = require('react/lib/ExecutionEnvironment').canUseDOM;
+
+var History = {
+
+  /**
+   * Sends the browser back one entry in the history, if one is available.
+   */
+  back: function () {
+    invariant(
+      canUseDOM,
+      'Cannot use History.back without a DOM'
+    );
+
+    // Do this first so that History.length will
+    // be accurate in location change listeners.
+    History.length -= 1;
+
+    window.history.back();
+  },
+
+  /**
+   * The current number of entries in the history.
+   */
+  length: 1
+
+};
+
+module.exports = History;

modules/utils/__tests__/History-test.js

@@ -0,0 +1,63 @@
+var expect = require('expect');
+var React = require('react');
+var { Foo, RedirectToFoo } = require('../../__tests__/TestHandlers');
+var TestLocation = require('../../locations/TestLocation');
+var Route = require('../../components/Route');
+var Router = require('../../index');
+var History = require('../History');
+
+describe('History', function () {
+  describe('on the initial page load', function () {
+    it('has length 1', function () {
+      expect(History.length).toEqual(1);
+    });
+  });
+
+  describe('after navigating to a route', function () {
+    beforeEach(function () {
+      TestLocation.history = [ '/foo' ];
+    });
+
+    it('has length 2', function (done) {
+      var routes = [
+        <Route name="foo" handler={Foo}/>,
+        <Route name="about" handler={Foo}/>
+      ];
+
+      var count = 0;
+
+      var router = Router.run(routes, TestLocation, function (Handler) {
+        count += 1;
+
+        if (count === 2) {
+          expect(History.length).toEqual(2);
+          done();
+        }
+      });
+
+      router.transitionTo('about');
+    });
+
+    describe('that redirects to another route', function () {
+      it('has length 2', function (done) {
+        var routes = [
+          <Route name="foo" handler={Foo}/>,
+          <Route name="about" handler={RedirectToFoo}/>
+        ];
+
+        var count = 0;
+
+        var router = Router.run(routes, TestLocation, function (Handler) {
+          count += 1;
+
+          if (count === 2) {
+            expect(History.length).toEqual(2);
+            done();
+          }
+        });
+
+        router.transitionTo('about');
+      });
+    });
+  });
+});

modules/utils/createRouter.js

@@ -18,6 +18,7 @@ var supportsHistory = require('./supportsHistory');
 var Transition = require('./Transition');
 var PropTypes = require('./PropTypes');
 var Redirect = require('./Redirect');
+var History = require('./History');
 var Cancellation = require('./Cancellation');
 var Path = require('./Path');
 
@@ -242,15 +243,23 @@ function createRouter(options) {
       },
 
       /**
-       * Transitions to the previous URL.
+       * Transitions to the previous URL. Returns true if the router
+       * was able to go back, false otherwise.
        */
       goBack: function () {
         invariant(
           typeof location !== 'string',
           'You cannot use goBack with a static location'
         );
 
-        location.pop();
+        if (History.length > 1) {
+          location.pop();
+          return true;
+        }
+
+        warning(false, 'goBack() was ignored because there is no router history');
+
+        return false;
       },
 
       /**