HTML5 Server-Sent Events: new EventSource as variable

I am working with ESP8266 moduls in Wifi-Station Mode. Each new ESP8266 module is assigned a different IP address by the wifi router. That’s why I need the Ip addresses as Javascript variable, but that doesn’t work:

    let ip = location.host;
    let str1 = 'http://';
    let str3 = '/see';
    
    let strIpAdress = str1+ip+str3;                  // doesn't work
    //let strIpAdress = 'http://192.168.178.47/sse'; // works!

    var source = new EventSource(strIpAdress); 
    console.log(strIpAdress);                        // http://192.168.178.47/sse

I have also tried in JavaScript String Object Format, but also without success.
Does a fixed IP address always have to be specified to prevent manipulation?
Is there a solution?