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## 1. What `"use strict"` is

> `"use strict"` is a directive that turns on **strict mode interpretation of JavaScript**.
>
> It helps you write **safer, more predictable code** by eliminating "silent" errors and old, non-obvious behavior.

Strict mode applies **within the file or function** where it is declared at the top:

```javascript
"use strict";
```

or:

```javascript
function foo() {
  "use strict";
  // strict mode only inside this function
}
```

---

## 2. The main change - the behavior of `this`

| Context | Without `"use strict"` | With `"use strict"` |
|---|---|---|
| In a regular function | `this` → `window` (in a browser) | `this` → `undefined` |
| In an object method | `this` → the object itself | `this` → the same object (unchanged) |
| In an arrow function | inherited from outside | inherited from outside (unchanged) |

---

### Example without strict mode

```javascript
function showThis() {
  console.log(this);
}

showThis(); // window (in a browser)
```

Here JS "substitutes" the global object (`window`) by default.

---

### Example with strict mode

```javascript
"use strict";

function showThis() {
  console.log(this);
}

showThis(); // undefined
```

In strict mode, if a function is called **not as a method of an object**,
`this` stays `undefined` - no implicit "magic" bindings.

This helps avoid accidental references to the global object (such as `window` or `global`).

---

## 3. Strict mode forbids implicit variable creation

Without strict mode:

```javascript
function test() {
  x = 10; // the variable gets created globally!
  console.log(x);
}

test();
console.log(window.x); // 10
```

With `"use strict"`:

```javascript
"use strict";

function test() {
  x = 10; // ReferenceError: x is not defined
}
test();
```

Now you must **declare variables explicitly** (`let`, `const`, `var`),
otherwise you get an error.

---

## 4. Forbids duplicate variable and parameter names

Without strict mode:

```javascript
function sum(a, a, c) {
  return a + a + c;
}

console.log(sum(1, 2, 3)); // 7
```

With `"use strict"`:

```javascript
"use strict";
function sum(a, a, c) { // SyntaxError
  return a + a + c;
}
```

This prevents mistakes and makes the code more predictable.

---

## 5. Forbids deleting variables, functions, and parameters

Without strict mode:

```javascript
var a = 1;
delete a; // false
```

With `"use strict"`:

```javascript
"use strict";
var a = 1;
delete a; // SyntaxError
```

In strict mode, you cannot delete anything that is not a property of an object.

---

## 6. Forbids assigning a value to `NaN`, `undefined`, `Infinity`

```javascript
"use strict";

undefined = 5; // TypeError
NaN = 123; // TypeError
Infinity = 42; // TypeError
```

---

## 7. Improves the safety of `eval`

Without strict mode, `eval` can create variables in the current context:

```javascript
eval("var x = 10;");
console.log(x); // 10
```

With strict mode, `eval` creates its own scope:

```javascript
"use strict";
eval("var x = 10;");
console.log(x); // ReferenceError
```

This prevents accidental pollution of the environment.

---

## 8. Protects against implicit "soft" errors

| Situation | Without `"use strict"` | With `"use strict"` |
|---|---|---|
| Assigning to `NaN`, `undefined`, `Infinity` | silently ignored | error |
| Assigning to `const` | ignored | error |
| The `with` statement | allowed | not allowed |
| Deleting properties from objects | silently ignored | error |

---

## SUMMARY

| What `"use strict"` changes | Behavior |
|---|---|
| `this` in regular functions | `undefined` instead of `window` |
| Undeclared variables | Error |
| Duplicate parameters | Error |
| Deleting variables/functions | Error |
| Assigning to `NaN`, `undefined`, `Infinity` | Error |
| `eval` | Creates its own scope |
| `with` | Forbidden |

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