The property '${identifier}' is not defined or is not readab

Error message

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

What it means

Thrown in the JSCallType.GetValue branch when isReadableProperty(parent, memberName) is false. isReadableProperty (line 652) walks the prototype chain checking descriptors: it returns false if the property is not 'in' the object, has no getter/setter and no value, or the descriptor lacks a 'value'/'get'. The message covers both 'not defined' and 'not readable' so the dev inspects the descriptor.

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) {

View on GitHub (pinned to 294cab2f9b)

Solutions

Verify the property exists on the instance or its prototype chain (prop in obj).
Check the descriptor via Object.getOwnPropertyDescriptor across the chain; add a getter/value if it's setter-only.
Ensure polyfills that define the property have loaded before the read.
Wrap the interop call in try/catch and provide a default.

Example fix

// before
await JS.InvokeAsync<object>("element.somethingWriteOnly");

// after
// expose a readable accessor in a shim
export function getSafe(o, k) {
  const desc = Object.getOwnPropertyDescriptor(o, k) ?? {};
  return ('value' in desc || desc.get) ? o[k] : undefined;
}

Defensive patterns

Strategy: type-guard

Validate before calling

// shim: safely read
export function readProp(obj, id) {
  const desc = Object.getOwnPropertyDescriptor(obj, id) ?? {};
  return ('value' in desc || !!desc.get) ? obj[id] : undefined;
}

Type guard

function isReadable(o:any, k:string):boolean {
  if (!(k in o)) return false;
  while (o) { const d = Object.getOwnPropertyDescriptor(o, k); if (d) return 'value' in d || !!d.get; o = Object.getPrototypeOf(o); }
  return false;
}

Try / catch

try { await JS.InvokeAsync('el.prop'); } catch (e) { if (/not defined or is not readable/i.test(e.message)) { return undefined; } throw e; }

Prevention

Check the property descriptor before relying on reads.
Ensure polyfills that define the property have loaded.
Prefer accessor wrapper functions over direct property interop.

When it happens

Trigger: Reading a property that does not exist on the object; reading a property defined only by a setter (write-only); reading a Symbol-keyed property the walker cannot see; reading a property whose getter throws is not this error (that throws the getter's error), but a property with only a setter is.

Common situations: Accessing a property added later by a polyfill that hasn't loaded; typo; version mismatch; trying to read an internal property removed between releases; write-only DOM properties.

Related errors

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/447f861e1a135504. Report an issue: GitHub.