Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

src/display/annotation_layer.js

@@ -81,9 +81,8 @@ AnnotationElementFactory.prototype =
               return new RadioButtonWidgetAnnotationElement(parameters);
             } else if (parameters.data.checkBox) {
               return new CheckboxWidgetAnnotationElement(parameters);
-            } else {
-              warn('Unimplemented button widget annotation: pushbutton');
             }
+            warn('Unimplemented button widget annotation: pushbutton');
             break;
           case 'Ch':
             return new ChoiceWidgetAnnotationElement(parameters);

src/display/api.js

@@ -1945,9 +1945,8 @@ var PDFObjects = (function PDFObjectsClosure() {
 
       if (!objs[objId]) {
         return false;
-      } else {
-        return objs[objId].resolved;
       }
+      return objs[objId].resolved;
     },
 
     hasData: function PDFObjects_hasData(objId) {
@@ -1961,9 +1960,8 @@ var PDFObjects = (function PDFObjectsClosure() {
       var objs = this.objs;
       if (!objs[objId] || !objs[objId].resolved) {
         return null;
-      } else {
-        return objs[objId].data;
       }
+      return objs[objId].data;
     },
 
     clear: function PDFObjects_clear() {

src/display/font_loader.js

@@ -376,7 +376,7 @@ var FontFaceObject = (function FontFaceObjectClosure() {
           this.options.fontRegistry.registerFont(this);
         }
         return nativeFontFace;
-      } else {
+      } else { // eslint-disable-line no-else-return
         throw new Error('Not implemented: createNativeFontFace');
       }
     },