dotnet/aspnetcore · error · Error

The property ' ' is not defined or is not readable.

Error message

The property '${identifier}' is not defined or is not readable.

What it means

Thrown by wrapJSCallAsFunction() in the GetValue case when isReadableProperty() returns false for the named member. A GetValue operation (.NET reading a property/value via interop) requires the property to exist and either be a data property or have a getter. Missing properties, non-function getters, or absent members all trigger this.

Solutions

1. Confirm the property exists: 'missingProp' in window.myObj and Object.getOwnPropertyDescriptor(window.myObj, 'missingProp').
2. Fix the identifier spelling / update to the new API name.
3. Ensure any lazy-initialized properties are set before the interop read.

Example fix

// before (C#)
var v = await JS.InvokeAsync<string>("myObj.titel"); // typo

// after
var v = await JS.InvokeAsync<string>("myObj.title");

function isReadable(obj, prop) { if (!(prop in obj)) return false; let o = obj; while (o) { const d = Object.getOwnPropertyDescriptor(o, prop); if (d) return d.hasOwnProperty('value') || (typeof d.get === 'function'); o = Object.getPrototypeOf(o); } return false; }

function isReadableMember(obj: any, prop: string): boolean { if (!(prop in obj)) return false; let o: any = obj; while (o) { const d = Object.getOwnPropertyDescriptor(o, prop); if (d) return d.hasOwnProperty('value') || typeof d.get === 'function'; o = Object.getPrototypeOf(o); } return false; }

try { var v = await JS.InvokeAsync<string>("myObj.title"); } catch (JSException e) when (e.Message.Contains("is not defined or is not readable")) { /* property missing/typo; fix identifier */ }

When it happens

Trigger: .NET calls JS.InvokeAsync<T>('myObj.missingProp') to read a value, but missingProp is not present on myObj or its prototype chain. Also when the property exists only as a setter (write-only) or has a non-function getter descriptor.

Common situations: Typo in the property name; property renamed in a JS library version; conditional property that isn't initialized yet at call time; accessing a property defined only with a setter.

Related errors

• Could not find ' ' (' ' was undefined).
• The property ' ' is not writable.
• The value ' ' is not a function.
• Byte array index ' ' does not exist.
• Cannot create a JSObjectReference from the value

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/447f861e1a135504. Report an issue: GitHub.

Appendix: source

Thrown at src/JSInterop/Microsoft.JSInterop.JS/src/src/Microsoft.JSInterop.ts:641

      switch (callType) {
      case JSCallType.FunctionCall:
          const func = parent[memberName];
          if (func instanceof Function) {
              return func.bind(parent);
          }
          throw new Error(`The value '${identifier}' is not a function.`);

      case JSCallType.ConstructorCall:
          const ctor = parent[memberName];
          if (ctor instanceof Function) {
              const bound = ctor.bind(parent);
              return (...args: any[]) => new bound(...args);
          }
          throw new Error(`The value '${identifier}' is not a function.`);

      case JSCallType.GetValue:
          if (!isReadableProperty(parent, memberName)) {
              throw new Error(`The property '${identifier}' is not defined or is not readable.`);
          }
          return () => parent[memberName];
      case JSCallType.SetValue:
          if (!isWritableProperty(parent, memberName)) {
              throw new Error(`The property '${identifier}' is not writable.`);
          }
          return (...args: any[]) => parent[memberName] = args[0];
      }
  }

  function isReadableProperty(obj: any, propName: string) {
      // Return false for missing property.
      if (!(propName in obj)) {
          return false;
      }

      // If the property is present we examine its descriptor, potentially needing to walk up the prototype chain.
      while (obj !== undefined) {

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