{"record":{"id":"07f117cccab1cabe","repo":"wasmerio/wasmer","slug":"unable-to-cast-to-a","errorCode":null,"errorMessage":"Unable to cast to a {}","messagePattern":"Unable to cast to a (.+?)","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"lib/wasix/src/http/web_http_client.rs","lineNumber":263,"sourceCode":"\n        header_map.insert(key, value);\n    }\n\n    Ok(header_map)\n}\n\nfn js_array<T, const N: usize>(value: &JsValue) -> Result<[T; N], anyhow::Error>\nwhere\n    T: JsCast,\n{\n    let array: &js_sys::Array = value.dyn_ref().context(\"Not an array\")?;\n\n    let mut items = Vec::new();\n\n    for value in array.iter() {\n        let item = value\n            .dyn_into()\n            .map_err(|_| anyhow::anyhow!(\"Unable to cast to a {}\", std::any::type_name::<T>()))?;\n        items.push(item);\n    }\n\n    <[T; N]>::try_from(items).map_err(|original| {\n        anyhow::anyhow!(\n            \"Unable to turn a list of {} items into an array of {N} items\",\n            original.len()\n        )\n    })\n}\n\npub async fn get_response_data(resp: &web_sys::Response) -> Result<Vec<u8>, anyhow::Error> {\n    let buffer = JsFuture::from(resp.array_buffer().unwrap())\n        .await\n        .map_err(js_error)\n        .with_context(|| \"Could not retrieve response body\".to_string())?;\n\n    let buffer = js_sys::Uint8Array::new(&buffer);","sourceCodeStart":245,"sourceCodeEnd":281,"githubUrl":"https://github.com/wasmerio/wasmer/blob/8c4b9ee9d33fb2068863fbb3d328683e7e6ff7f5/lib/wasix/src/http/web_http_client.rs#L245-L281","documentation":"js_array in lib/wasix/src/http/web_http_client.rs converts a JS array-like (e.g. the result of Array.from(headers)) into a fixed-size Rust array [T; N] by dyn_into-casting each element. If any element is not actually of type T (expected pairs of [name, value] arrays but found something else), the dyn_into fails and this error names the expected Rust type.","triggerScenarios":"The JS value iterated in `headers` contains elements that don't match the expected shape — e.g. Headers.entries() returned items that aren't length-2 string arrays, or the JS bridge returned objects instead of arrays.","commonSituations":"Running in a JS environment whose Headers implementation returns non-standard iteration results (polyfills, workers, Node vs browser differences); a header entry containing null/undefined values; mismatch between expected tuple arity and what the host returns.","solutions":["Check the inner JS environment: run in a standard browser with native Headers support, not a partial polyfill.","Log/inspect the raw JS array before conversion to see the actual element shape.","Ensure the response/request headers are plain string-to-string pairs; convert non-string values with String() on the JS side.","Update the CLI/runtime — host binding mismatches between JS glue and Rust expectations are often fixed upstream.","If you control the JS glue, normalize entries to [string, string] arrays before passing them across the boundary."],"exampleFix":"// before (JS glue passes headers entries verbatim)\nconst entries = headers.entries();\nfetchBytes(entries);\n// after: normalize to [string, string] pairs\nconst entries = Array.from(headers.entries())\n  .map(([k, v]) => [String(k), String(v)])\n  .filter(([k, v]) => k != null && v != null);\nfetchBytes(entries);","handlingStrategy":"type-guard","validationCode":"// normalize the JS array shape before it reaches js_array\nconst entries = Array.from(headers.entries())\n  .filter(e => Array.isArray(e) && e.length === 2\n      && typeof e[0] === 'string' && typeof e[1] === 'string')\n  .map(([k, v]) => [String(k), String(v)]);","typeGuard":"// Rust-side narrowing check before dyn_into\nfn is_js_string_pair(v: &JsValue) -> bool {\n    js_sys::Array::is_array(v)\n        && js_sys::Array::from(v).length() == 2\n        && js_sys::Array::from(v).get(0).is_string()\n        && js_sys::Array::from(v).get(1).is_string()\n}","tryCatchPattern":"match headers(client).await {\n    Err(e) if e.to_string().contains(\"Unable to cast to a\") => {\n        eprintln!(\"{e}\\nHint: JS headers entries are not [string, string] pairs — normalize on the JS side or update the runtime\");\n        std::process::exit(1);\n    }\n    other => other,\n}","preventionTips":["Test in a standard browser with native Headers support before exotic environments","Coerce header keys/values to strings on the JS side before crossing the wasm boundary","Keep the JS glue and Rust runtime versions in sync","Log raw JS values when debugging bridging failures instead of guessing the shape"],"tags":["wasm","javascript","type-cast","headers","wasm-bindgen"],"backgroundTag":"js-type-cast-failed","analyzedSha":"8c4b9ee9d33fb2068863fbb3d328683e7e6ff7f5","analyzedAt":"2026-09-01T23:06:31.009Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-09T06:17:21.866Z"}