Because they were using a demo function from a JS lib that attempts to match error messages: https://github.com/facebookincubator/idx https://github.com/facebo

Because they were using a demo function from a JS lib that attempts to match error messages: https://github.com/facebookincubator/idx https://github.com/facebookincubator/idx >The idx runtime function exists for the purpose of illustrating the expected behavior and is not meant to be executed. The idx function requires the use of a Babel plugin that replaces it with an implementation that does not depend on details related to browser error messages. https://github.com/facebookincubator/idx/issues/55 https://github.com/facebookincubator/idx/issues/55