LayoutTests/fetch/fetch-error-messages.html

<!DOCTYPE html>
<script src='https://github.com/resources/testharness.js'></script>
<script src='https://github.com/resources/testharnessreport.js'></script>
<script>
function printMethodError(method, target)
{
    try {
        method.call(target);
        assert_unreached();
    } catch(e) {
         console.log(e);
    }
}

function printPromiseMethodError(method, target)
{
    return method.call(target).then(assert_unreached, (e) => {
         console.log("Promise rejected with: " + e);
    });
}

function printGetterError(object, getterName, target)
{
    const getter = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(object), getterName).get;
    printMethodError(getter, target);
}

promise_test(function(test) {
    // This test prints exceptions to check the format of their messages.

    var request = new Request("");
    var response = new Response("");

    var results = [
        printMethodError(response.clone, request),
        printPromiseMethodError(response.arrayBuffer, request),
        printPromiseMethodError(response.blob, request),
        printPromiseMethodError(response.formData, request),
        printPromiseMethodError(response.json, request),
        printPromiseMethodError(response.text, request),
        printGetterError(response, "body", request),
        printGetterError(request, "method", response),
        printMethodError(request.clone, response),
    ];
    return Promise.all(results);
}, "Exercising TypeError messages in Fetch Request and Response");
</script>