jestjs/jest · error · Error

No property name supplied

Error message

No property name supplied

What it means

Thrown by jest.spyOn when the second argument (methodKey) is null or undefined. The spy needs the name of the property to intercept, so the guard rejects a missing name immediately.

Solutions

1. Pass the property name as a string literal or a defined variable.
2. Check that any variable used as the key is defined before the spyOn call.
3. Search the codebase for usages of that key to confirm the intended name.

Example fix

// before
jest.spyOn(obj, mathodName); // typo -> undefined
// after
jest.spyOn(obj, 'methodName');

if (methodKey == null) {
  throw new TypeError('spyOn requires a property name');
}
jest.spyOn(obj, methodKey);

const hasMethodName = (k: unknown): k is string | number | symbol =>
  k != null;

When it happens

Trigger: jest.spyOn(obj); jest.spyOn(obj, undefined); jest.spyOn(obj, methodName) where methodName is an undefined variable.

Common situations: Refactor removed the method but left the spyOn call; a typo in the variable name; a computed key built from an undefined value; forgetting the second argument entirely.

Related errors

• Cannot replace the ` ` property because it is a function…
• Cannot spy on the ` ` property because it is not a…
• Cannot spy on the property because it is not a function…
• Cannot use spyOn on a primitive value;
• Property ` ` does not exist in the provided object

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/5a142810594a690a. Report an issue: GitHub.

Appendix: source

Thrown at packages/jest-mock/src/index.ts:1286

    methodKey: K,
  ): V extends ClassLike | FunctionLike ? Spied<V> : never;

  spyOn<T extends object>(
    object: T,
    methodKey: keyof T,
    accessType?: 'get' | 'set',
  ): MockInstance {
    if (
      object == null ||
      (typeof object !== 'object' && typeof object !== 'function')
    ) {
      throw new Error(
        `Cannot use spyOn on a primitive value; ${this._typeOf(object)} given`,
      );
    }

    if (methodKey == null) {
      throw new Error('No property name supplied');
    }

    if (accessType) {
      return this._spyOnProperty(object, methodKey, accessType);
    }

    const original = object[methodKey];

    if (!original) {
      throw new Error(
        `Property \`${String(
          methodKey,
        )}\` does not exist in the provided object`,
      );
    }

    if (!this.isMockFunction(original)) {
      if (typeof original !== 'function') {

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