How to call typescript function from external javascript function?

Ionic2 project. I want to call typescript funtion from external js file. js file is based on tomcat server. I used inappbrowser when I connect js file. Here is my typescript code.

constructor(public navCtrl: NavController, private iab: InAppBrowser, private elementRef : ElementRef) {
var ref = in here/, ‘_self’, ‘location=yes’);
function getGPS(e):string { return “client getGPS func” ;}

I want to call getGPS function.
Here is my javascript code. First, html code.

//input text box
GPS button // id=“GPSBtn” onclick="umn2.getGPS()"GPS

Second, javascript code.

getGPS: function(){
console.log(“getGPS function”);
document.getElementById(“resultArea”).value = //want to call client function result

How can I call getGPS fucntion in typescript from js function?
Please tell me. Thank you!