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An ESLint rule is a separate, checkable condition that the code must satisfy. Each rule is responsible for a **specific aspect of quality, style, or safety**, and when it is violated, ESLint reports an error or a warning.

---

### **What a rule does**

A rule can:

- **forbid** a dangerous or unwanted construct
(for example, `no-eval`, `no-unused-vars`)
- **enforce a consistent style**
(for example, `quotes` - single or double quotes)
- **warn about potential bugs**
(for example, `eqeqeq` - use `===` instead of `==`)

---

### **What a rule looks like in the config**

In `.eslintrc`, each rule has a level:

```json
{
  "rules": {
    "no-unused-vars": "warn",
    "eqeqeq": "error",
    "quotes": ["error", "single"]
  }
}
```

Where `"off" | "warn" | "error"` means:

| Level | What it means |
|---|---|
| `"off"` | the rule is disabled |
| `"warn"` | a warning, but the build does not fail |
| `"error"` | an error - it affects the build/commit |

---

### **An important point**

ESLint is a set of *rules*, not one general requirement. Each rule is responsible for its own area of the code, and together they form the project's **code style and quality rules**.

---

### **Summary**

> **An ESLint rule is a specific check that controls a certain behavior or style in the code and signals its violation.**

Through a set of such rules, ESLint ensures cleanliness, predictability, and consistency of code in a project.

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