var rrule = "DTSTART=20190514T111500Z;FREQ=DAILY;INTERVAL=1";

[
  dtstart: 20190514T111500Z,
  freq: daily,
  interval: 1
]

var array = rrule.split(";");

[
  "DTSTART=20190514T111500Z",
  "FREQ=DAILY",
  "INTERVAL=1"
]

let str = "DTSTART=20190514T111500Z;FREQ=DAILY;INTERVAL=1";

let obj = {};
for (let entry of str.split(";")) {
    let pair = entry.split("=");
    obj[pair[0]] = pair[1];
}

console.log(obj);

{DTSTART: "20190514T111500Z", FREQ: "DAILY", INTERVAL: "1"}