weird-string-case.js

function toWeirdCase(string, isUpperCase = true){
  return string.split('').reduce((weirdCase, letter) => {
    if (letter !== ' ') {
      if (isUpperCase) {
        weirdCase += letter.toUpperCase();
      } else {
        weirdCase += letter.toLowerCase();
      }
      isUpperCase = !isUpperCase;
    } else {
      weirdCase += ' ';
      isUpperCase = true;
    }
    
    return weirdCase;
  }, '');
}

function toWeirdCase(string, isUpperCase = true){
  return string.split('').reduce((weirdCase, letter) => {
    if (letter !== ' ') {
      if (isUpperCase) {
        isUpperCase = !isUpperCase;
        return weirdCase + letter.toUpperCase();
      } else {
        isUpperCase = !isUpperCase;
        return weirdCase + letter.toLowerCase();
      }
    } else {
      isUpperCase = true;
      return weirdCase + ' ';
    }
  }, '');
}

function toWeirdCase(string, isUpperCase = true){
  return string.split('').reduce((weirdCase, letter) => {
    if (letter !== ' ') {
      if (isUpperCase) {
        isUpperCase = !isUpperCase;
        return weirdCase + letter.toUpperCase();
      } else {
        isUpperCase = !isUpperCase;
        return weirdCase + letter.toLowerCase();
      }
    } else {
      isUpperCase = true;
      return weirdCase + ' ';
    }
  }, '');
}

console.log(toWeirdCase('This'), 'ThIs');
console.log(toWeirdCase('is'), 'Is');
console.log(toWeirdCase('This is a test'), 'ThIs Is A TeSt');