fix: clarify comments.

src/model/Component.ts

@@ -47,11 +47,15 @@ const inner = makeInner<{
 
 class ComponentModel<Opt extends ComponentOption = ComponentOption> extends Model<Opt> {
 
-    // [Caution]: for compat the previous "class extend"
-    // publich and protected fields must be initialized on
-    // prototype rather than in constructor. Otherwise the
-    // subclass overrided filed will be overwritten by this
-    // class. That is, they should not be initialized here.
+    // [Caution]: Becuase this class or desecendants can be used as `XXX.extend(subProto)`,
+    // the class members must not be initialized in constructor or declaration place.
+    // Otherwise there is bad case:
+    //   class A {xxx = 1;}
+    //   enableClassExtend(A);
+    //   class B extends A {}
+    //   var C = B.extend({xxx: 5});
+    //   var c = new C();
+    //   console.log(c.xxx); // expect 5 but always 1.
 
     /**
      * @readonly

src/model/Model.ts

@@ -46,11 +46,15 @@ type Value<Opt, R> = Opt extends Dictionary<any>
 
 class Model<Opt extends ModelOption = ModelOption> {    // TODO: TYPE use unkown insteadof any?
 
-    // [Caution]: for compat the previous "class extend"
-    // publich and protected fields must be initialized on
-    // prototype rather than in constructor. Otherwise the
-    // subclass overrided filed will be overwritten by this
-    // class. That is, they should not be initialized here.
+    // [Caution]: Becuase this class or desecendants can be used as `XXX.extend(subProto)`,
+    // the class members must not be initialized in constructor or declaration place.
+    // Otherwise there is bad case:
+    //   class A {xxx = 1;}
+    //   enableClassExtend(A);
+    //   class B extends A {}
+    //   var C = B.extend({xxx: 5});
+    //   var c = new C();
+    //   console.log(c.xxx); // expect 5 but always 1.
 
     /**
      * @readOnly

src/model/Series.ts

@@ -109,11 +109,15 @@ interface SeriesModel {
 
 class SeriesModel<Opt extends SeriesOption = SeriesOption> extends ComponentModel<Opt> {
 
-    // [Caution]: for compat the previous "class extend"
-    // publich and protected fields must be initialized on
-    // prototype rather than in constructor. Otherwise the
-    // subclass overrided filed will be overwritten by this
-    // class. That is, they should not be initialized here.
+    // [Caution]: Becuase this class or desecendants can be used as `XXX.extend(subProto)`,
+    // the class members must not be initialized in constructor or declaration place.
+    // Otherwise there is bad case:
+    //   class A {xxx = 1;}
+    //   enableClassExtend(A);
+    //   class B extends A {}
+    //   var C = B.extend({xxx: 5});
+    //   var c = new C();
+    //   console.log(c.xxx); // expect 5 but always 1.
 
     // @readonly
     type: string;

src/view/Chart.ts

@@ -95,11 +95,15 @@ interface ChartView {
 }
 class ChartView {
 
-    // [Caution]: for compat the previous "class extend"
-    // publich and protected fields must be initialized on
-    // prototype rather than in constructor. Otherwise the
-    // subclass overrided filed will be overwritten by this
-    // class. That is, they should not be initialized here.
+    // [Caution]: Becuase this class or desecendants can be used as `XXX.extend(subProto)`,
+    // the class members must not be initialized in constructor or declaration place.
+    // Otherwise there is bad case:
+    //   class A {xxx = 1;}
+    //   enableClassExtend(A);
+    //   class B extends A {}
+    //   var C = B.extend({xxx: 5});
+    //   var c = new C();
+    //   console.log(c.xxx); // expect 5 but always 1.
 
     // @readonly
     type: string;

src/view/Component.ts

@@ -45,11 +45,15 @@ interface ComponentView {
 
 class ComponentView {
 
-    // [Caution]: for compat the previous "class extend"
-    // publich and protected fields must be initialized on
-    // prototype rather than in constructor. Otherwise the
-    // subclass overrided filed will be overwritten by this
-    // class. That is, they should not be initialized here.
+    // [Caution]: Becuase this class or desecendants can be used as `XXX.extend(subProto)`,
+    // the class members must not be initialized in constructor or declaration place.
+    // Otherwise there is bad case:
+    //   class A {xxx = 1;}
+    //   enableClassExtend(A);
+    //   class B extends A {}
+    //   var C = B.extend({xxx: 5});
+    //   var c = new C();
+    //   console.log(c.xxx); // expect 5 but always 1.
 
     readonly group: ViewRootGroup;