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A mock function (or *mock*) is **a fake function that substitutes for the real one in tests**, letting you control its behavior and check how the tested code interacts with it.

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### **Why it is needed**

Mock functions are used when:

- calling the real function is **undesirable** (it makes a network request, hits a database, writes a file, calls a third-party service)
- you need to check the **fact of the call**, not the result (for example, "is the function called, and with which arguments?")
- you need to **emulate different scenarios** (success, error, delay, empty response)
- the function under test **depends on others**, and those dependencies need to be isolated

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### **What a mock function can do**

It lets you:

1. **Replace the real call**
2. **Supply the needed response** (for example, `return 42`)
3. **Track calls**: how many times it was called, with what arguments, in what order
4. **Check interaction**, not computation

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### **A simple example in Jest**

```js
const log = jest.fn(); // mock function

log("hello");

expect(log).toHaveBeenCalled();           // check that the function was called at all
expect(log).toHaveBeenCalledWith("hello"); // check that it was called with the right arguments
```

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### **The main idea**

**A mock function is not about the result, it is about behavior.**
It lets you test the logic of the current piece of code without depending on "what is happening out there."

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