camelCase and undoCamelCase

camelCase.js

function camelCase (string, separator) {
    return string.split(separator).map((word, index) => word.substr(0,1)[index ? 'toUpperCase' : 'toLowerCase']() + word.substr(1) ).join('');
}

console.log(camelCase('metal gear solid', ' '));
// metalGearSolid

function undoCamelCase (string, separator) {
	return string.split('').map((letter) => letter.toLowerCase() !== letter ? separator + letter.toLowerCase() : letter).join('');
}

console.log(undoCamelCase('helloWorld', '_'));
// => hello_world