jestjs/jest · error · Error

${propertyName} is not declared configurable

Error message

${propertyName} is not declared configurable

What it means

Thrown by `SpyRegistry._spyOnProperty` in spyRegistry.ts:178 when the descriptor's `configurable` flag is false. jasmine2 redefines the property with `Object.defineProperty`, which throws on non-configurable properties; this guard gives a clearer message first.

Source

Thrown at packages/jest-jasmine2/src/jasmine/spyRegistry.ts:178

      }

      if (!propertyName) {
        throw new Error(getErrorMsg('No property name supplied'));
      }

      let descriptor: PropertyDescriptor | undefined;
      try {
        descriptor = Object.getOwnPropertyDescriptor(obj, propertyName);
      } catch {
        // IE 8 doesn't support `definePropery` on non-DOM nodes
      }

      if (!descriptor) {
        throw new Error(getErrorMsg(`${propertyName} property does not exist`));
      }

      if (!descriptor.configurable) {
        throw new Error(
          getErrorMsg(`${propertyName} is not declared configurable`),
        );
      }

      if (!descriptor[accessType]) {
        throw new Error(
          getErrorMsg(
            `Property ${propertyName} does not have access type ${accessType}`,
          ),
        );
      }

      if (obj[propertyName] && isSpy(obj[propertyName])) {
        if (this.respy) {
          return obj[propertyName];
        } else {
          throw new Error(
            getErrorMsg(`${propertyName} has already been spied upon`),

View on GitHub (pinned to f49721c78e)

Solutions

1. Make the property configurable in production code (`Object.defineProperty(obj, 'p', { configurable: true, ... })`).
2. Spy on the prototype if the instance property is non-configurable but the prototype property is configurable.
3. Refactor to inject the dependency so you do not need to redefine a frozen property.

Example fix

// before
spyOnProperty(frozenConfig, 'timeout', 'get') // non-configurable
// after
// make it configurable in the module under test, or inject the value
spyOnProperty(Proto.prototype, 'timeout', 'get')

const d = Object.getOwnPropertyDescriptor(obj, propertyName);
if (d && !d.configurable) { throw new Error(`${propertyName} is non-configurable`); }
spyOnProperty(obj, propertyName, 'get');

const isConfigurable = (o: object, p: string): boolean => {
  const d = Object.getOwnPropertyDescriptor(o, p);
  return !d || Boolean(d.configurable);
};

When it happens

Trigger: Spying on a property defined with `configurable: false`, a non-configurable built-in (e.g. `window.location` props), or an object sealed/frozen with `Object.seal`/`Object.freeze`.

Common situations: Frozen config objects; sealed singletons; TS `readonly` fields compiled non-configurable; trying to spy on host-provided non-configurable globals.

Related errors

• ${methodName} is not declared writable or has no setter
• could not find an object to spy upon for ${propertyName}
• No property name supplied
• ${propertyName} property does not exist
• Property ${propertyName} does not have access type ${accessT

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/47cddda507bbe05e.json. Report an issue: GitHub.