dotnet/runtime · error · Error

${part} not found while looking up ${function_name}

Error message

${part} not found while looking up ${function_name}

What it means

Thrown by mono_wasm_lookup_js_import when walking the dotted segments of a [JSImport]-annotated function name and an intermediate property is not present on the scope object. The runtime splits the function name on '.' and traverses each intermediate part; if any non-final segment is undefined, it throws before even checking whether the final name is callable.

Source

Thrown at src/mono/browser/runtime/invoke-js.ts:404

        scope = importedModules.get(js_module_name);
        if (WasmEnableThreads) {
            mono_assert(scope, () => `ES6 module ${js_module_name} was not imported yet, please call JSHost.ImportAsync() on the UI or JSWebWorker thread first in order to invoke ${function_name}.`);
        } else {
            mono_assert(scope, () => `ES6 module ${js_module_name} was not imported yet, please call JSHost.ImportAsync() first in order to invoke ${function_name}.`);
        }
    } else if (parts[0] === "INTERNAL") {
        scope = INTERNAL;
        parts.shift();
    } else if (parts[0] === "globalThis") {
        scope = globalThis;
        parts.shift();
    }

    for (let i = 0; i < parts.length - 1; i++) {
        const part = parts[i];
        const newscope = scope[part];
        if (!newscope) {
            throw new Error(`${part} not found while looking up ${function_name}`);
        }
        scope = newscope;
    }

    const fname = parts[parts.length - 1];
    const fn = scope[fname];

    if (typeof (fn) !== "function") {
        throw new Error(`${function_name} must be a Function but was ${typeof fn}`);
    }

    // if the function was already bound to some object it would stay bound to original object. That's good.
    return fn.bind(scope);
}

export function set_property (self: any, name: string, value: any): void {
    mono_check(self, "Null reference");
    self[name] = value;

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Verify the JSImport attribute string exactly matches the export path in your JS module (check spelling and segment order).
2. Ensure you awaited JSHost.ImportAsync(moduleName, url) for the module before invoking the import.
3. Check the actual shape of the imported module (console.log the module object) to confirm the intermediate properties exist.
4. Flatten the export or fix the attribute to match the real structure.

Example fix

// before: JS exports a flat function but JSImport uses a nested path
// JS:  export function doThing() {}
// C#: [JSImport("myMod.Sub.doThing")] static partial void DoThing();

// after: match the real export shape
// C#: [JSImport("myMod.doThing")] static partial void DoThing();
// and import first: await JSHost.ImportAsync("myMod", "./myMod.js");

// Verify each intermediate segment exists before binding [JSImport]
function pathResolvesToScope(scope, dotted) {
  let s = scope;
  for (const part of dotted.split('.').slice(0, -1)) {
    if (s == null || !(part in s)) return false;
    s = s[part];
  }
  return s != null;
}
if (!pathResolvesToScope(myModule, jsImportName)) { /* fix the path or import the module */ }

function isImportPathValid(scope: any, dotted: string): boolean {
  let s = scope;
  for (const part of dotted.split('.').slice(0, -1)) {
    if (s == null || typeof s !== 'object' || !(part in s)) return false;
    s = s[part];
  }
  return true;
}

When it happens

Trigger: Produced when binding a [JSImport] whose name contains intermediate path segments that do not exist on the resolved scope. E.g. [JSImport("Foo.Bar.Baz")] when the imported module/scope has Foo but no Foo.Bar property.

Common situations: Typo in the JSImport string; renamed/moved a JS export but did not update the attribute; the JS module was not imported via JSHost.ImportAsync before the [JSImport] binding ran; mismatch between the dotted path and the actual export shape (e.g. flat export referenced via nested path).

Related errors

• ${part} not found while looking up ${functionName}
• ${function_name} must be a Function but was ${typeof fn}
• ${functionName} must be a Function but was ${typeof fn}
• Cannot call synchronous C# methods.
• Cannot call synchronous C# method from inside a synchronous

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/9b7dcebd9d58a96f. Report an issue: GitHub.