Mixpanel JQL returning indexOf is not a function

I have a JQL query in mixpanel which should return result based on two different events. Here are the steps:

1. First event after filtering would return some array of values as result.
2. Second event should use that result and return the required values after some filtering.

JQL query as below

function main() {
var date_range = {
    from_date: "2017-08-02",
    to_date: "2017-08-02"
};
var firstEventResult = Events(date_range).map(function(event) {
  return (event.name === "country" && event.properties.sector === "IT") ? event.properties.Area: null;
}).filter(function(v) {
    return v;
});
return Events(date_range).map(function(event) {
    return (event.name === "state" && firstEventResult.indexOf(event.properties.Area)) ? event.properties.message: null;
}).filter(function(v) {
     return v;
 });
}

In the above code, firstEventResult.indexOf() is returning error as "indexOf is not a function". But, firstEventResult is returning an array like below

[
  "Delhi"
  "Punjab"
  "Kerala"
]