I know this is an old answer, but this handles both whitespace and _ (lodash)
function toCamelCase(s){
return s
.replace(/_/g, " ")
.replace(/\s(.)/g, function($1) { return $1.toUpperCase(); })
.replace(/\s/g, '')
.replace(/^(.)/, function($1) { return $1.toLowerCase(); });
}
console.log(toCamelCase("Hello world");
console.log(toCamelCase("Hello_world");
// Both print "helloWorld"