Refactor reducers into a map of action type -> update function

Split the large switch statements in each reducer function
into objects which map action types to state update functions.

These functions are then composed together with a `createReducer`
helper defined in `reducers/util`

h/static/scripts/reducers/annotations.js

@@ -10,18 +10,7 @@ var metadata = require('../annotation-metadata');
 var uiConstants = require('../ui-constants');
 
 var selection = require('./selection');
-
-var actions = {
-  ADD_ANNOTATIONS: 'ADD_ANNOTATIONS',
-  REMOVE_ANNOTATIONS: 'REMOVE_ANNOTATIONS',
-  CLEAR_ANNOTATIONS: 'CLEAR_ANNOTATIONS',
-
-  /**
-   * Update an annotation's status flags after attempted anchoring in the
-   * document completes.
-   */
-  UPDATE_ANCHOR_STATUS: 'UPDATE_ANCHOR_STATUS',
-};
+var util = require('./util');
 
 /**
  * Return a copy of `current` with all matching annotations in `annotations`
@@ -87,10 +76,8 @@ function init() {
   };
 }
 
-function update(state, action) {
-  switch (action.type) {
-  case actions.ADD_ANNOTATIONS:
-    {
+var update = {
+  ADD_ANNOTATIONS: function (state, action) {
     var updatedIDs = {};
     var updatedTags = {};
 
@@ -128,29 +115,29 @@ function update(state, action) {
       }
     });
 
-      return Object.assign({}, state, {
-        annotations: added.concat(updated).concat(unchanged),
-      });
-    }
-  case actions.REMOVE_ANNOTATIONS:
-    {
+    return {annotations: added.concat(updated).concat(unchanged)};
+  },
+
+  REMOVE_ANNOTATIONS: function (state, action) {
     var annots = excludeAnnotations(state.annotations, action.annotations);
     var selectedTab = state.selectedTab;
     if (selectedTab === uiConstants.TAB_ORPHANS &&
         arrayUtil.countIf(annots, metadata.isOrphan) === 0) {
       selectedTab = uiConstants.TAB_ANNOTATIONS;
     }
+
+    var tabUpdateFn = selection.update.SELECT_TAB;
     return Object.assign(
-        {},
-        state,
       {annotations: annots},
-        selection.selectTabHelper(state, selectedTab)
+      tabUpdateFn(state, selection.selectTab(selectedTab))
     );
-    }
-  case actions.CLEAR_ANNOTATIONS:
-    return Object.assign({}, state, {annotations: []});
-  case actions.UPDATE_ANCHOR_STATUS:
-    {
+  },
+
+  CLEAR_ANNOTATIONS: function () {
+    return {annotations: []};
+  },
+
+  UPDATE_ANCHOR_STATUS: function (state, action) {
     var annotations = state.annotations.map(function (annot) {
       var match = (annot.id && annot.id === action.id) ||
                   (annot.$$tag && annot.$$tag === action.tag);
@@ -163,12 +150,11 @@ function update(state, action) {
         return annot;
       }
     });
-      return Object.assign({}, state, {annotations: annotations});
-    }
-  default:
-    return state;
-  }
-}
+    return {annotations: annotations};
+  },
+};
+
+var actions = util.actionTypes(update);
 
 /** Add annotations to the currently displayed set. */
 function addAnnotations(annotations, now) {

h/static/scripts/reducers/index.js

@@ -31,11 +31,11 @@ function init(settings) {
   );
 }
 
-var update = util.composeReducers([
+var update = util.createReducer(Object.assign(
   annotations.update,
   selection.update,
-  viewer.update,
-]);
+  viewer.update
+));
 
 module.exports = {
   init: init,

h/static/scripts/reducers/selection.js

@@ -14,6 +14,8 @@ var immutable = require('seamless-immutable');
 
 var uiConstants = require('../ui-constants');
 
+var util = require('./util');
+
 /**
 * Default starting tab.
 */
@@ -58,43 +60,6 @@ function toSet(list) {
   }, {});
 }
 
-/**
- * Return state updates necessary to select a different tab.
- *
- * This function accepts the name of a tab and returns an object which must be
- * merged into the current state to achieve the desired tab change.
- */
-function selectTabHelper(state, newTab) {
-  // Do nothing if the "new tab" is not a valid tab.
-  if ([uiConstants.TAB_ANNOTATIONS,
-      uiConstants.TAB_NOTES,
-      uiConstants.TAB_ORPHANS].indexOf(newTab) === -1) {
-    return {};
-  }
-  // Shortcut if the tab is already correct, to avoid resetting the sortKey
-  // unnecessarily.
-  if (state.selectedTab === newTab) {
-    return {};
-  }
-  return {
-    selectedTab: newTab,
-    sortKey: TAB_SORTKEY_DEFAULT[newTab],
-    sortKeysAvailable: TAB_SORTKEYS_AVAILABLE[newTab],
-  };
-}
-
-var actions = {
-  CLEAR_SELECTION: 'CLEAR_SELECTION',
-  SELECT_ANNOTATIONS: 'SELECT_ANNOTATIONS',
-  FOCUS_ANNOTATIONS: 'FOCUS_ANNOTATIONS',
-  HIGHLIGHT_ANNOTATIONS: 'HIGHLIGHT_ANNOTATIONS',
-  SET_FORCE_VISIBLE: 'SET_FORCE_VISIBLE',
-  SET_EXPANDED: 'SET_EXPANDED',
-  SET_FILTER_QUERY: 'SET_FILTER_QUERY',
-  SET_SORT_KEY: 'SET_SORT_KEY',
-  SELECT_TAB: 'SELECT_TAB',
-};
-
 function init(settings) {
   return {
     // Contains a map of annotation tag:true pairs.
@@ -127,37 +92,64 @@ function init(settings) {
   };
 }
 
-function update(state, action) {
-  switch (action.type) {
-  case actions.CLEAR_SELECTION:
-    return Object.assign({}, state, {
-      filterQuery: null,
-      selectedAnnotationMap: null,
-    });
-  case actions.SELECT_ANNOTATIONS:
-    return Object.assign({}, state, {selectedAnnotationMap: action.selection});
-  case actions.FOCUS_ANNOTATIONS:
-    return Object.assign({}, state, {focusedAnnotationMap: action.focused});
-  case actions.SET_FORCE_VISIBLE:
-    return Object.assign({}, state, {forceVisible: action.forceVisible});
-  case actions.SET_EXPANDED:
-    return Object.assign({}, state, {expanded: action.expanded});
-  case actions.HIGHLIGHT_ANNOTATIONS:
-    return Object.assign({}, state, {highlighted: action.highlighted});
-  case actions.SELECT_TAB:
-    return Object.assign({}, state, selectTabHelper(state, action.tab));
-  case actions.SET_FILTER_QUERY:
-    return Object.assign({}, state, {
+var update = {
+  CLEAR_SELECTION: function () {
+    return {filterQuery: null, selectedAnnotationMap: null};
+  },
+
+  SELECT_ANNOTATIONS: function (state, action) {
+    return {selectedAnnotationMap: action.selection};
+  },
+
+  FOCUS_ANNOTATIONS: function (state, action) {
+    return {focusedAnnotationMap: action.focused};
+  },
+
+  SET_FORCE_VISIBLE: function (state, action) {
+    return {forceVisible: action.forceVisible};
+  },
+
+  SET_EXPANDED: function (state, action) {
+    return {expanded: action.expanded};
+  },
+
+  HIGHLIGHT_ANNOTATIONS: function (state, action) {
+    return {highlighted: action.highlighted};
+  },
+
+  SELECT_TAB: function (state, action) {
+    // Do nothing if the "new tab" is not a valid tab.
+    if ([uiConstants.TAB_ANNOTATIONS,
+        uiConstants.TAB_NOTES,
+        uiConstants.TAB_ORPHANS].indexOf(action.tab) === -1) {
+      return {};
+    }
+    // Shortcut if the tab is already correct, to avoid resetting the sortKey
+    // unnecessarily.
+    if (state.selectedTab === action.tab) {
+      return {};
+    }
+    return {
+      selectedTab: action.tab,
+      sortKey: TAB_SORTKEY_DEFAULT[action.tab],
+      sortKeysAvailable: TAB_SORTKEYS_AVAILABLE[action.tab],
+    };
+  },
+
+  SET_FILTER_QUERY: function (state, action) {
+    return {
       filterQuery: action.query,
       forceVisible: {},
       expanded: {},
-    });
-  case actions.SET_SORT_KEY:
-    return Object.assign({}, state, {sortKey: action.key});
-  default:
-    return state;
-  }
-}
+    };
+  },
+
+  SET_SORT_KEY: function (state, action) {
+    return {sortKey: action.key};
+  },
+};
+
+var actions = util.actionTypes(update);
 
 function select(annotations) {
   return {
@@ -325,7 +317,4 @@ module.exports = {
   // Selectors
   hasSelectedAnnotations: hasSelectedAnnotations,
   isAnnotationSelected: isAnnotationSelected,
-
-  // Helpers
-  selectTabHelper: selectTabHelper,
 };

h/static/scripts/reducers/util.js

 'use strict';
 
 /**
- * Compose a list of `(state, action) => new state` reducer functions
- * into a single reducer.
+ * Return an object where each key in `updateFns` is mapped to the key itself.
  */
-function composeReducers(reducers) {
+function actionTypes(updateFns) {
+  return Object.keys(updateFns).reduce(function (types, key) {
+    types[key] = key;
+    return types;
+  }, {});
+}
+
+/**
+ * Given an object which maps action names to update functions, this returns
+ * a reducer function that can be passed to the redux `createStore` function.
+ */
+function createReducer(updateFns) {
   return function (state, action) {
-    return reducers.reduce(function (state, reducer) {
-      return reducer(state, action);
-    }, state);
+    var fn = updateFns[action.type];
+    if (!fn) {
+      return state;
+    }
+    return Object.assign({}, state, fn(state, action));
   };
 }
 
 module.exports = {
-  composeReducers: composeReducers,
+  actionTypes: actionTypes,
+  createReducer: createReducer,
 };

h/static/scripts/reducers/viewer.js

 'use strict';
 
+var util = require('./util');
+
 /**
  * This module defines actions and state related to the display mode of the
  * sidebar.
  */
 
-var actions = {
-  /** Sets whether annotated text is highlighted in the page. */
-  SET_HIGHLIGHTS_VISIBLE: 'SET_HIGHLIGHTS_VISIBLE',
-
-  /**
-   * Set whether the app is the sidebar or not.
-   *
-   * When not in the sidebar, we do not expect annotations to anchor and always
-   * display all annotations, rather than only those in the current tab.
-   */
-  SET_SIDEBAR: 'SET_SIDEBAR',
-};
-
 function init() {
   return {
     // Flag that indicates whether the app is the sidebar and connected to
@@ -28,16 +17,16 @@ function init() {
   };
 }
 
-function update(state, action) {
-  switch (action.type) {
-  case actions.SET_SIDEBAR:
-    return Object.assign({}, state, {isSidebar: action.isSidebar});
-  case actions.SET_HIGHLIGHTS_VISIBLE:
-    return Object.assign({}, state, {visibleHighlights: action.visible});
-  default:
-    return state;
-  }
-}
+var update = {
+  SET_SIDEBAR: function (state, action) {
+    return {isSidebar: action.isSidebar};
+  },
+  SET_HIGHLIGHTS_VISIBLE: function (state, action) {
+    return {visibleHighlights: action.visible};
+  },
+};
+
+var actions = util.actionTypes(update);
 
 /** Set whether the app is the sidebar */
 function setAppIsSidebar(isSidebar) {