#3562·yew

Tutorial proxy-backend returning `index.html` not json?

Author: ydirsonCreated Dec 17, 2023Updated Apr 8, 2026

Note: I think I followed the tuto right, but it may well be a user error - what really happens behind my back in trunk serve --proxy-backend indeed worries me.

Problem In the last step of the tutorial, the trunk serve process does report proxying as expected:

2023-12-16T23:28:47.916146Z  INFO  proxying /tutorial -> https://yew.rs/tutorial

but the call to json() on the Response object fails with a parse error, caught by unwrap():

panicked at src/main.rs:74:22:
called `Result::unwrap()` on an `Err` value: SerdeError(Error("expected value", line: 1, column: 1))

When querying its text() instead I get something looking like dist/index.html, excep:

with template bits like {{__TRUNK_ADDRESS__}} expanded
with different filenames, but always the 56dc19c1d... hash in them, whereas the one in dist changes on every rebuild

"<!DOCTYPE html><html lang=\"en\"><head>\n<link rel=\"preload\" href=\"/yew-tuto-56dc19c1d2397a2a9d7ed57de056ad246e9780a244ab777c8af3501c5b435ab460786da12a03b5d575a5b77bb0c0b951_bg.wasm\" as=\"fetch\" type=\"application/wasm\" crossorigin=\"anonymous\" integrity=\"sha384-VtwZwdI5eiqdftV94FatJG6XgKJEq3d8ivNQHFtDWrRgeG2hKgO11XWlt3uwwLlR\">\n<link rel=\"modulepreload\" href=\"/yew-tuto-56dc19c1d2397a2a9d7ed57de056ad246e9780a244ab777c8af3501c5b435ab460786da12a03b5d575a5b77bb0c0b951.js\" crossorigin=\"anonymous\" integrity=\"sha384-abm7hNroHgmO-4dhhx1owgizoubsoronoQebv8kzdegbKJXclKG1Se4cNkK9b48k\"></head>\n    <body>\n\n\n<script type=\"module\">\nimport init, * as bindings from '/yew-tuto-56dc19c1d2397a2a9d7ed57de056ad246e9780a244ab777c8af3501c5b435ab460786da12a03b5d575a5b77bb0c0b951.js';\ninit('/yew-tuto-56dc19c1d2397a2a9d7ed57de056ad246e9780a244ab777c8af3501c5b435ab460786da12a03b5d575a5b77bb0c0b951_bg.wasm');\nwindow.wasmBindings = bindings;\n\n</script><script>\"use strict\";\n\n(function () {\n\n    const address = 'localhost:8080';\n    let protocol = '';\n    protocol =\n        protocol\n

when my index.html reads:

<!DOCTYPE html><html lang="en"><head>
<link rel="preload" href="/yew-tuto-dfbd59d34641e15def5e2ad596d898d62fbcc1fa07f7416e23f9e3fcac1b96cac82ac4ddd0b1765a62a49daf68df933a_bg.wasm" as="fetch" type="application/wasm" crossorigin="anonymous" integrity="sha384-371Z00ZB4V3vXirVltiY1i-8wfoH90FuI_nj_KwblsrIKsTd0LF2WmKkna9o35M6">
<link rel="modulepreload" href="/yew-tuto-dfbd59d34641e15def5e2ad596d898d62fbcc1fa07f7416e23f9e3fcac1b96cac82ac4ddd0b1765a62a49daf68df933a.js" crossorigin="anonymous" integrity="sha384-T7wdMh576Bm4zh1ka8x7ATfiVk0hQi1dd3noLX_rCbtJmfmE_W9PnMYJdEjnGnIW"></head>
    <body>


<script type="module">
import init, * as bindings from '/yew-tuto-dfbd59d34641e15def5e2ad596d898d62fbcc1fa07f7416e23f9e3fcac1b96cac82ac4ddd0b1765a62a49daf68df933a.js';
init('/yew-tuto-dfbd59d34641e15def5e2ad596d898d62fbcc1fa07f7416e23f9e3fcac1b96cac82ac4ddd0b1765a62a49daf68df933a_bg.wasm');
window.wasmBindings = bindings;

</script><script>"use strict";

(function () {

    const address = '{{__TRUNK_ADDRESS__}}';
    let protocol = '';
...

curl http://127.0.0.1:8080/tutorial/data.json does dump me the expected JSON (and I also tried to add an intermediate tutorial step just for decoding, see repo, but I'm not pleased with how it diverges too much to be useful)

Steps To Reproduce

I uploaded my steps to https://github.com/ydirson/test-yew-tutorial

Steps to reproduce the behavior:

checkout commit 'use a proxy server' to see the .json().unwrap() fail
checkout commit DBG to see the text() dump

Environment:

Yew version:

version = "0.21.0"
source = "git+https://github.com/yewstack/yew/#1d889347a6205a2dc79c1d1fea9d49092c0627db"

Rust version: 1.74.0
Target: wasm32-unknown-unknown
Build tool, if relevant: trunk
OS, if relevant: Linux (Debian 12, amd64)
Browser and version: Firefox 115.5.0esr

Questionnaire

I'm interested in fixing this myself but don't know where to start
I would like to fix and I have a solution
I don't have time to fix this right now, but maybe later

Source: yewstack/yew

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