Web: Improve opaque network error message

ehttp/src/streaming/web.rs

@@ -4,7 +4,7 @@ use futures_util::Stream;
 use futures_util::StreamExt;
 use wasm_bindgen::prelude::*;
 
-use crate::web::{fetch_base, get_response_base, spawn_future, string_from_js_value};
+use crate::web::{fetch_base, get_response_base, spawn_future, string_from_fetch_error};
 use crate::Request;
 
 use super::types::Part;
@@ -19,8 +19,8 @@ pub async fn fetch_async_streaming(
 ) -> crate::Result<impl Stream<Item = crate::Result<Part>>> {
     let stream = fetch_jsvalue_stream(request)
         .await
-        .map_err(string_from_js_value)?;
-    Ok(stream.map(|result| result.map_err(string_from_js_value)))
+        .map_err(string_from_fetch_error)?;
+    Ok(stream.map(|result| result.map_err(string_from_fetch_error)))
 }
 
 #[cfg(feature = "streaming")]
@@ -54,7 +54,7 @@ pub(crate) fn fetch_streaming(
         let mut stream = match fetch_jsvalue_stream(&request).await {
             Ok(stream) => stream,
             Err(e) => {
-                on_data(Err(string_from_js_value(e)));
+                on_data(Err(string_from_fetch_error(e)));
                 return;
             }
         };
@@ -67,7 +67,7 @@ pub(crate) fn fetch_streaming(
                     }
                 }
                 Err(e) => {
-                    on_data(Err(string_from_js_value(e)));
+                    on_data(Err(string_from_fetch_error(e)));
                     return;
                 }
             }

ehttp/src/web.rs

@@ -9,11 +9,23 @@ use crate::{Request, Response};
 /// NOTE: `Ok(…)` is returned on network error.
 /// `Err` is only for failure to use the fetch API.
 pub async fn fetch_async(request: &Request) -> crate::Result<Response> {
-    fetch_jsvalue(request).await.map_err(string_from_js_value)
+    fetch_jsvalue(request)
+        .await
+        .map_err(string_from_fetch_error)
 }
 
-pub(crate) fn string_from_js_value(value: JsValue) -> String {
-    value.as_string().unwrap_or_else(|| format!("{:#?}", value))
+/// This should only be used to handle opaque exceptions thrown by the `fetch` call.
+pub(crate) fn string_from_fetch_error(value: JsValue) -> String {
+    value.as_string().unwrap_or_else(|| {
+        // TypeError means that this is an opaque `network error`, as defined by the spec:
+        // https://fetch.spec.whatwg.org/
+        if value.has_type::<js_sys::TypeError>() {
+            web_sys::console::error_1(&value);
+            "Failed to fetch, check the developer console for details".to_owned()
+        } else {
+            format!("{:#?}", value)
+        }
+    })
 }
 
 pub(crate) async fn fetch_base(request: &Request) -> Result<web_sys::Response, JsValue> {