var url = "https://en.wikipedia.org/w/api.php"; 

var params = {
    action: "compare",
    format: "json",
    fromtitle: "Template:Unsigned",
    totitle: "Template:UnsignedIP",
    origin: "*"

};
fetch(url + "?" + (new URLSearchParams(params)).toString())
    .then(resp => resp.json())
    .then(data => {
        console.log("data", data);
    })
    .catch(err => {
        console.log("bad", err);
    });