How to check unwrapError - mithril.js

Question

var users = m.request({
method: "GET",
url: "hoge.json",
unwrapSuccess: function(response) {
return response;
},
unwrapError: function(response) {
//return response.error;
return "404 error";
}
});
users.then(function(result) {
console.log(result);
});
After delete "hoge.json".
I want to catch "404 error",but
uncaught SyntaxError: Unexpected token <
2016/2/18 add
I want to test alert ("unwrapError");
Below code is always alert ("unwrapSuccess");
How to change below code?
What is the unwrapError?
▼js
var users = m.request({
method: "GET",
url: "hoge.json",
unwrapSuccess: function(response) {
alert ("unwrapSuccess");
return response;
},
unwrapError: function(response) {
alert ("unwrapError");
return "error";
}
});
users.then(function(result) {
console.log(result);
});
▼hoge.json
[{"name": "John"}, {"name": "Mary"}]

Answer 1

If you take a look at mithril's source code you will see that m.request is just a wrapper for the XMLHttpRequest API. And that's what happens when the request's readyState attribute changes:
xhr.onreadystatechange = function () {
if (xhr.readyState === 4) {
if (xhr.status >= 200 && xhr.status < 300) {
options.onload({type: "load", target: xhr})
} else {
options.onerror({type: "error", target: xhr})
}
}
}
So mithril's unwrapError callback will be called whenever the response status is not a 2xx.
I updated the fiddle calling a URL that returns a 500 response and now the unwrapError is called.