clean up selected lines reducer

components/Carbon.js

@@ -388,16 +388,22 @@ function selectedLinesReducer(
 ) {
   const newState = {}
 
-  if (type === 'GROUP' && prevLine) {
+  switch (type) {
+    case 'GROUP': {
+      if (prevLine) {
         for (let i = Math.min(prevLine, lineNumber); i < Math.max(prevLine, lineNumber) + 1; i++) {
           newState[i] = selected[prevLine]
         }
       }
-  if (type === 'MULTILINE') {
+      break
+    }
+    case 'MULTILINE': {
       for (let i = 0; i < selectedLines.length; i++) {
         newState[selectedLines[i] - 1] = true
       }
-  } else {
+      break
+    }
+    default: {
       for (let i = 0; i < numLines; i++) {
         if (i != lineNumber) {
           if (prevLine == null) {
@@ -408,6 +414,7 @@ function selectedLinesReducer(
         }
       }
     }
+  }
 
   return {
     selected: { ...selected, ...newState },
@@ -441,11 +448,8 @@ function useSelectedLines(props, editorRef) {
 
   return React.useCallback(function onGutterClick(editor, lineNumber, gutter, e) {
     const numLines = editor.display.view.length
-    if (e.shiftKey) {
-      dispatch({ type: 'GROUP', lineNumber, numLines })
-    } else {
-      dispatch({ type: 'LINE', lineNumber, numLines })
-    }
+    const type = e.shiftKey ? 'GROUP' : 'LINE'
+    dispatch({ type, lineNumber, numLines })
   }, [])
 }