jestjs/jest · error · TypeError

Cannot spy on the ${String(propertyKey)} property because it

Error message

Cannot spy on the ${String(propertyKey)} property because it is not a function; ${this._typeOf(original)} given instead.${typeof original === 'object' ? '' : ` If you are trying to mock a property, use `jest.replaceProperty(object, '${String(propertyKey)}', value)` instead.`}

What it means

Thrown by _spyOnProperty (as a TypeError) when the located accessor is truthy but typeof !== 'function' (and not already a mock). Per the JS spec a descriptor's get/set are function or undefined, so reaching this branch implies a manually corrupted/tampered descriptor. It is a defensive guard; the message also suggests jest.replaceProperty for non-object originals.

Source

Thrown at packages/jest-mock/src/index.ts:1411

    if (!descriptor.configurable) {
      throw new Error(
        `Property \`${String(propertyKey)}\` is not declared configurable`,
      );
    }

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

    const original = descriptor[accessType];

    if (!this.isMockFunction(original)) {
      if (typeof original !== 'function') {
        throw new TypeError(
          `Cannot spy on the ${String(
            propertyKey,
          )} property because it is not a function; ${this._typeOf(
            original,
          )} given instead.${
            typeof original === 'object'
              ? ''
              : ` If you are trying to mock a property, use \`jest.replaceProperty(object, '${String(
                  propertyKey,
                )}', value)\` instead.`
          }`,
        );
      }

      descriptor[accessType] = this._makeComponent({type: 'function'}, () => {
        if (isPropertyOwner) {
          // @ts-expect-error: mock is assignable
          descriptor![accessType] = original;

View on GitHub (pinned to 8e6d128e4a)

Solutions

1. Inspect Object.getOwnPropertyDescriptor(obj, key) and confirm get/set are functions.
2. Remove any code that mutates descriptors in a non-standard way.
3. Restore the property via Object.defineProperty with a proper function accessor before spying.

Example fix

// before (descriptor tampered)
Object.defineProperty(obj, 'x', {get: 42 as any, configurable: true});
jest.spyOn(obj, 'x', 'get');
// after
Object.defineProperty(obj, 'x', {get: () => 42, configurable: true});
jest.spyOn(obj, 'x', 'get');

const d = Object.getOwnPropertyDescriptor(obj, propertyKey);
if (!d || typeof d[accessType] !== 'function') {
  throw new Error(`${String(propertyKey)} ${accessType} accessor is not a function`);
}
jest.spyOn(obj, propertyKey, accessType);

const accessorIsFunction = (o: object, k: PropertyKey, a: 'get' | 'set'): boolean => {
  const d = Object.getOwnPropertyDescriptor(o, k);
  return !!d && typeof d[a] === 'function';
};

When it happens

Trigger: Manually assigning descriptor.get = 'notAFunction'; a custom Proxy returning a malformed descriptor; a third-party library mutating descriptors in a non-standard way.

Common situations: Extremely rare; only appears with non-standard descriptor manipulation or exotic Proxy traps. Normal code never hits this.

Related errors

• Property `${String(propertyKey)}` does not exist in the prov
• Property `${String(propertyKey)}` is not declared configurab
• Property `${String(propertyKey)}` does not have access type
• ${RECEIVED_COLOR('received')} value must be a mock or spy fu
• ${RECEIVED_COLOR('received')} value must be a mock function

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/c752f9aafdba360d. Report an issue: GitHub.