Avoid mutating blockAttributes

packages/blocks/src/api/parser/fix-custom-classname.js

@@ -42,27 +42,30 @@ export function getHTMLRootElementClasses( innerHTML ) {
  * @return {Object} Filtered block attributes.
  */
 export function fixCustomClassname( blockAttributes, blockType, innerHTML ) {
-	if ( hasBlockSupport( blockType, 'customClassName', true ) ) {
-		// To determine difference, serialize block given the known set of
-		// attributes, with the exception of `className`. This will determine
-		// the default set of classes. From there, any difference in innerHTML
-		// can be considered as custom classes.
-		const { className: omittedClassName, ...attributesSansClassName } =
-			blockAttributes;
-		const serialized = getSaveContent( blockType, attributesSansClassName );
-		const defaultClasses = getHTMLRootElementClasses( serialized );
-		const actualClasses = getHTMLRootElementClasses( innerHTML );
+	if ( ! hasBlockSupport( blockType, 'customClassName', true ) ) {
+		return blockAttributes;
+	}
+
+	const modifiedBlockAttributes = { ...blockAttributes };
+	// To determine difference, serialize block given the known set of
+	// attributes, with the exception of `className`. This will determine
+	// the default set of classes. From there, any difference in innerHTML
+	// can be considered as custom classes.
+	const { className: omittedClassName, ...attributesSansClassName } =
+		modifiedBlockAttributes;
+	const serialized = getSaveContent( blockType, attributesSansClassName );
+	const defaultClasses = getHTMLRootElementClasses( serialized );
+	const actualClasses = getHTMLRootElementClasses( innerHTML );
 
-		const customClasses = actualClasses.filter(
-			( className ) => ! defaultClasses.includes( className )
-		);
+	const customClasses = actualClasses.filter(
+		( className ) => ! defaultClasses.includes( className )
+	);
 
-		if ( customClasses.length ) {
-			blockAttributes.className = customClasses.join( ' ' );
-		} else if ( serialized ) {
-			delete blockAttributes.className;
-		}
+	if ( customClasses.length ) {
+		modifiedBlockAttributes.className = customClasses.join( ' ' );
+	} else if ( serialized ) {
+		delete modifiedBlockAttributes.className;
 	}
 
-	return blockAttributes;
+	return modifiedBlockAttributes;
 }