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Expectations in a test are described using the construct:

```javascript
expect(actualResult).matcher(expectedValue);
```

It compares what the function returned with what was supposed to happen.

---

### **The simplest example**

```js
expect(sum(2, 3)).toBe(5);
```

Here:

- `expect(sum(2, 3))` - the actual result
- `.toBe(5)` - the expectation (matcher) that the result should equal `5`

---

### **The most common matchers**

| Matcher | What it checks |
| --- | --- |
| `toBe(value)` | strict equality (like `===`) |
| `toEqual(obj)` | compares objects and arrays by content |
| `toBeTruthy()` | the value is truthy |
| `toBeFalsy()` | the value is falsy |
| `toBeNull()` | strictly `null` |
| `toBeUndefined()` | strictly `undefined` |
| `toContain(item)` | the item is present in an array or string |
| `toBeGreaterThan(num)` | greater than… |
| `toBeLessThan(num)` | less than… |

---

### **Example with different expectations**

```js
expect([1, 2, 3]).toContain(2);
expect({ name: "Tom" }).toEqual({ name: "Tom" });
expect(10).toBeGreaterThan(5);
expect(false).toBeFalsy();
```

---

### **The meaning of expectations**

Every expectation is a statement that must be true.
If even one of them does not match reality, the test fails and shows exactly what went wrong.

---

In other words, expectations describe **how the code is supposed to behave**, while Jest automatically checks whether the actual behavior matches the expected one.

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