Change case kata

exercise.js

function changeCase(identifier, targetCase){
  if(!isSupportedCase(targetCase)) {
   return;
  }
  
  if(!isValid(identifier)) {
    return;
  }
  
  const words = tokenise(identifier);
  
  return format(targetCase, words);
}

function isSupportedCase(targetCase) {
  const supportedCases = ['camel', 'snake', 'kebab'];
  
  return supportedCases.includes(targetCase);
}

function isValid(identifier) {
  const camelSeparator = /[A-Z]/;
  const kebabSeparator = /_/;
  const snakeSeparator = /-/;
  
  const separators = [camelSeparator, kebabSeparator, snakeSeparator]
    .map(check => check.test(identifier))
    .filter(v => v)
    .length;
  
  return separators <= 1;
}

function tokenise(identifier) {
  return identifier
    .split(/-|_|(?=[A-Z])/)
    .map(a => a.toLowerCase());
}

function format(targetCase, words) {
  switch(targetCase) {
    case "kebab":
      return words.join('-');
    case "snake":
      return words.join('_');
    case "camel":
      const [first, ...rest] = words;
      return [first, ...rest.map(capitalise)].join('');
  }
}

function capitalise(word) {
  return word.replace(/./, letter => letter.toUpperCase());
}