Use jest.spyOn()
- The method already exists on an object.
- You want to track calls to the real method.
- You may want the original implementation to run.
- You may temporarily mock the method and restore it.
Jest mock comparison
jest.spyOn() vs jest.fn(): which one should you use?
Both can track calls and control mock behavior, but they start from different situations. Compare them side by side and choose the right Jest mock for your test.
jest.spyOn()
Watches a method that already exists and calls the original implementation by default.
jest.fn()
Creates a new mock function with no original implementation unless you provide one.
Quick answer
The main difference in one minute
Use jest.spyOn() when the method already exists
and you want Jest to observe or temporarily change that real
method.
Use jest.fn() when you need to create a new mock
function, callback, dependency or placeholder yourself.
Choose jest.spyOn() if...
You already have an object method and want to watch what happens when your code calls it.
Choose jest.fn() if...
You need a mock function that does not need to wrap an existing method.
Choose based on what already exists
The easiest way to decide is to look at the function you are trying to test.
Use jest.fn()
- You need to create a mock from scratch.
- You are mocking a callback.
- No real implementation needs to run.
- You want to inject a fake function into your code.
jest.spyOn() vs jest.fn() comparison
The important differences become clearer when you compare their behavior directly.
Feature
jest.spyOn()
jest.fn()
Requires an existing method
Yes
No
Tracks calls
Yes
Yes
Tracks arguments
Yes
Yes
Original code runs by default
Yes
No
Can use mockReturnValue()
Yes
Yes
Can use mockImplementation()
Yes
Yes
Can restore an original method
Yes
Not automatically
Best for callbacks
Usually no
Yes
See the difference in real code
These examples perform similar assertions, but the mock is created in a different way.
Existing method
jest.spyOn()
const service = {
send(message) {
return message;
}
};
const spy = jest.spyOn(
service,
'send'
);
service.send('hello');
expect(spy)
.toHaveBeenCalledWith('hello');
New mock function
jest.fn()
const send = jest.fn();
send('hello');
expect(send)
.toHaveBeenCalledWith(
'hello'
);
Why spyOn fits here
service.send() already exists, so the spy
watches that real method rather than creating a separate
function.
Why jest.fn fits here
No function existed yet. The test creates a new mock function and calls it directly.
Which one should you use in these cases?
Match your testing situation with the tool that fits it best.
Watch an existing API method
jest.spyOn()
The service method already exists and your application calls it.
Create a callback mock
jest.fn()
You need a fake callback to pass into another function.
Verify a real method was called
jest.spyOn()
You want to observe an existing object's behavior.
Inject a fake dependency
jest.fn()
Your code expects a function and you can supply the dependency yourself.
Mock temporarily, then restore
jest.spyOn()
You want the real object method back after the test.
Build a mock implementation from scratch
jest.fn()
There is no real object method that needs to be preserved.
A simple rule to remember
If the function already exists and belongs to an object,
jest.spyOn() is usually the clearer starting point.
If you need to create the function yourself, start with
jest.fn().
jest.spyOn() vs jest.fn() questions
Quick answers to the differences developers most often need to understand.
jest.spyOn() wraps a method that already
exists. jest.fn() creates a new mock
function from scratch.
Yes. By default the original implementation runs. You can replace the behavior by adding a mock return value or mock implementation.
Usually yes. A callback often does not need an existing
object method, so creating it directly with
jest.fn() is simpler.
A standalone jest.fn() does not have an
original object method to restore. A spy can restore
the method it wrapped.
Put the right Jest pattern into practice
Continue with the guide, examples or troubleshooting page that matches your next step.