j.js

function changeCase(identifier, targetCase){
  if(identifier.length === 0) { return "" }
  if(!targetCase.match(/^(snake|kebab|camel)$/)) { return undefined }    
  
  var replaceCapsWithStars = identifier.replace(/[A-Z]/g, '*')
  if(new Set(replaceCapsWithStars.match(/[_\-\*]/g)).size > 1) { return undefined }
  
  identifier = identifier.replace(/[A-Z]/g, (match) => '-' + match.toLowerCase())
  identifierArray = identifier.split(/[_\-]/g)
  
  if(targetCase === 'snake') { return identifierArray.join('_') }
  if(targetCase === 'kebab') { return identifierArray.join('-') }
  
  identifierArray = identifierArray.map(function(element, index) {
    if(index === 0) { return element }
    return element.replace(/[a-z]/, (match) => match.toUpperCase())
  })
  
  return identifierArray.join('')
}