Javascript question (switch, if, function...)

I am new here and wish you everything goes awesome!

This question is about t

I wanna get the result like

a is ok 

b is ok

c is changed

d is fail

e is ok

a is ok

b is ok

c is changed

d is fail

using these javascript

var i = 0;

function arrayList() {
  var list = ["a", "b", "c", "d", "e"];
  var result = list[i];

  i = i + 1;

  if (i === 5) {
    i = 0;
  }

  return result;
}

function switchCD(arrayListValue) {
  switch (arrayListValue) {
    case "c":
    case "d": {
      arrayListValue = arrayListValue + " is changed";
      console.log(arrayListValue);
      var switchCDValue = arrayList();
      return switchCDValue;
    }
  }
}

function failOk(switchCDValue) {
  if (switchCDValue === "c" || switchCDValue === "d") {
    switchCDValue = switchCDValue + " is fail";
    console.log(switchCDValue);
  } else {
    switchCDValue = switchCDValue + " is ok";
    console.log(switchCDValue);
  }

  return switchCDValue;
}

function loop() {
  for (var i = 0; i < 7; i++) {
    var test1 = arrayList();

    var test2 = switchCD(test1);

    failOk(test2);
  }
}

loop();

but I don't know where it go wrong here.

Please help me to figure it out.