Return proxy redirecting keys for options-as-properties from Command constructor

Borrowed from tj#1919.
Makes _optionValues the only true storage for option values.
Has the added benefit of supporting option names conflicting with
instance's properties even when options-as-properties are enabled.

Author: aweebit

lib/command.js

@@ -95,6 +95,67 @@ Option value deletion is not supported`);
 Options value configuration is not supported`);
       }
     });
+
+    // Because of how the returned proxy works, ideally, no prooerties should be defined outside the cinstructor.
+    // They can still be defined outside the constructor in subclasses, but only when _storeOptionsAsProperties is set to false.
+    this._version = undefined;
+    this._versionOptionName = undefined;
+
+    // The proxy only treats keys not present in the instance and its prototype chain as keys for _optionValues when _storeOptionsAsProperties is set to true.
+    // Setting option values for keys present in the instance and its prototype chain is still possible by calling .setOptionValue() or .setOptionValueWithSource(),
+    // but such values will not be accessible as instnace properties because the instance and its prototype chain has precedence.
+    // However, they will be accessible via .getOptionValue(), .opts() and .optsWithGlobals().
+    return new Proxy(this, {
+      get(target, key, receiver) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = receiver = receiver._optionValuesProxy;
+        }
+        return Reflect.get(target, key, receiver);
+      },
+      set(target, key, value, receiver) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = receiver = receiver._optionValuesProxy;
+        }
+        return Reflect.set(target, key, value, receiver);
+      },
+      has(target, key) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = target._optionValuesProxy;
+        }
+        return Reflect.has(target, key);
+      },
+      deleteProperty(target, key) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = target._optionValuesProxy;
+        }
+        return Reflect.deleteProperty(target, key);
+      },
+      defineProperty(target, key, descriptor) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = target._optionValuesProxy;
+        }
+        return Reflect.defineProperty(target, key, descriptor);
+      },
+      getOwnPropertyDescriptor(target, key) {
+        if (target._storeOptionsAsProperties && !(key in target)) {
+          target = target._optionValuesProxy;
+        }
+        return Reflect.getOwnPropertyDescriptor(target, key);
+      },
+      ownKeys(target) {
+        const result = Reflect.ownKeys(target);
+        if (target._storeOptionsAsProperties) {
+          result.push(...Reflect.ownKeys(target._optionValuesProxy));
+        }
+        return result;
+      },
+      preventExtensions(target) {
+        if (target._storeOptionsAsProperties) {
+          Reflect.preventExtensions(target._optionValuesProxy);
+        }
+        return Reflect.preventExtensions(target);
+      }
+    });
   }
 
   /**
@@ -783,9 +844,6 @@ Expecting one of '${allowedValues.join("', '")}'`);
    */
 
   getOptionValue(key) {
-    if (this._storeOptionsAsProperties) {
-      return this[key];
-    }
     return this._optionValues[key];
   }
 
@@ -811,11 +869,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     */
 
   setOptionValueWithSource(key, value, source) {
-    if (this._storeOptionsAsProperties) {
-      this[key] = value;
-    } else {
-      this._optionValues[key] = value;
-    }
+    this._optionValues[key] = value;
     this._optionValueSources[key] = source;
     return this;
   }
@@ -1580,7 +1634,9 @@ Expecting one of '${allowedValues.join("', '")}'`);
 
       for (let i = 0; i < len; i++) {
         const key = this.options[i].attributeName();
-        result[key] = key === this._versionOptionName ? this._version : this[key];
+        result[key] = key === this._versionOptionName
+          ? this._version
+          : this._optionValues[key];
       }
       return result;
     }