It's ok to clear the outdir.

CMakeLists.txt

@@ -6,7 +6,7 @@ find_package(ament_cmake REQUIRED)
 add_custom_target(build ALL)
 add_custom_command(TARGET build
                    POST_BUILD
-                   COMMAND yarn build --outDir ${CMAKE_INSTALL_PREFIX}/share/${PROJECT_NAME}/dist
+                   COMMAND yarn build --emptyOutDir --outDir ${CMAKE_INSTALL_PREFIX}/share/${PROJECT_NAME}/dist
                    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
 )