jestjs/jest · error · TypeError

Argument passed to callFake should be a function, got

Error message

Argument passed to callFake should be a function, got ${fn}

What it means

A spy's `.callFake(fn)` replaces the spy's behavior with the given function. SpyStrategy.callFake validates that `fn` is a function at SpyStrategy.ts:91; if not, it throws a TypeError showing the actual value passed. Passing a non-function would silently break the spy's stub behavior, so jasmine fails fast.

Solutions

Pass the function reference, not its invocation: `spy.and.callFake(myFn)` (no parentheses).
If the fake should be async, pass an async function: `spy.and.callFake(async () => {...})`.
Check that the variable holding the function is defined before passing it.

Example fix

// before
spy.and.callFake(realImpl());
// after
spy.and.callFake(realImpl);

Defensive patterns

Strategy: type-guard

Validate before calling

// Guard callFake calls with a type check.
function safeCallFake(spy: { and: { callFake: (fn: Function) => unknown } }, fn: unknown) {
  if (typeof fn !== 'function') {
    throw new TypeError(`callFake requires a function, got ${typeof fn}`);
  }
  return spy.and.callFake(fn);
}

Type guard

function isCallable(value: unknown): value is Function {
  return typeof value === 'function';
}

Prevention

Pass the function reference, not its return value: `callFake(fn)` not `callFake(fn())`.
For async fakes, pass an async arrow function.
Verify the function variable is defined before passing it.

When it happens

Trigger: Calling `spy.and.callFake('not a function')`, `spy.and.callFake(undefined)`, or `spy.and.callFake({})` where a function was expected. The check `typeof fn !== 'function'` at line 91 triggers the throw at line 92.

Common situations: Passing the return value of a function instead of the function itself (e.g. `callFake(myFn())` instead of `callFake(myFn)`); passing an async value or a promise; a variable that was conditionally assigned and is undefined; refactoring that changed a function reference to an object.

Related errors

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/94db76f62a1e6af4. Report an issue: GitHub.

Appendix: source

Thrown at packages/jest-jasmine2/src/jasmine/SpyStrategy.ts:92

      const values = Array.prototype.slice.call(arguments);
      plan = function () {
        return values.shift();
      };
      return getSpy();
    };

    this.throwError = function (something) {
      const error =
        something instanceof Error ? something : new Error(something);
      plan = function () {
        throw error;
      };
      return getSpy();
    };

    this.callFake = function (fn) {
      if (typeof fn !== 'function') {
        throw new TypeError(
          `Argument passed to callFake should be a function, got ${fn}`,
        );
      }
      plan = fn;
      return getSpy();
    };

    this.stub = function (_fn) {
      plan = function () {};
      return getSpy();
    };
  }
}

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