Change snake to camel, camel to snake, uppercase and lowercase functions

changeCase.js

function snakeToCamel(str) {
	return str.replace(/(\_[a-z])/g, function(char){return char[1].toUpperCase();});
}

function camelToSnake(str) {
  return str.replace(/([A-Z])/g, function(char){return "_" + char.toLowerCase();});
}

function upperCaseFirst(str) {
  return str[0].toUpperCase() + str.slice(1);
}

function lowerCaseFirst(str) {
  return str[0].toLowerCase() + str.slice(1);
}

function changeCase(str) {
  return str.replace(/[A-Za-z]/g, function(char) {
    var upper = char.toUpperCase();
    return upper === char ? upper : char.toLowerCase();
  });
}