30-seconds-of-code/test/fromCamelCase/fromCamelCase.js

module.exports = (str, separator = '_') =>
str
.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
.replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
.toLowerCase();