Convert PascalCase string into snake_case.js

/*
Description:
Complete the function/method so that it takes CamelCase string and returns the string in snake_case notation. Lowercase characters can be numbers. If method gets number, it should return string.

Examples:

//  returns test_controller
toUnderscore('TestController');

// returns movies_and_books
toUnderscore('MoviesAndBooks');

// returns app7_test
toUnderscore('App7Test');

// returns "1"
toUnderscore(1);
*/

function toUnderscore(string) {
  return (''+string).replace(/(.)([A-Z])/g, '$1_$2').toLowerCase();
}