How to get ISO-8859-1 encoded string?

Question

I am working on pass authorization header to encoded text. Server expects ISO-8851-1 encoding format.

Here I used iconv and try to get encoded string and it gives a different encoded string . what can be the reason?

var str = this.username+':'+this.password;
var icBufeer = iconv.encode(str, 'ISO-8859-1');
var auth = btoa(icBufeer .toString());
headers.append('Authorization', 'Basic ' + auth );