## `inject()` Function

### Description

The `inject()` function in DTL is designed for merging a flattened object into a deeply nested object. It facilitates the precise addition or modification of data within complex data structures.

### Usage

```plaintext
inject($object $flattened_object)
```

- `$object`: The original object into which data will be injected.
- `$flattened_object`: An object with keys in dot notation representing the paths for injection.

### Details

`inject()` takes two parameters: the original object and a flattened object. The flattened object's keys should use dot notation to specify where the values need to be placed in the original object. If the path specified by a key does not exist in the original object, `inject()` will create the necessary nested structures (objects or arrays) to accommodate the value.

### Example

Given an object:

```json
{
  "user": {
    "name": "John",
    "age": 30
  }
}
```

And a flattened object:

```json
{
  "user.address.street": "123 Main St",
  "user.age": 31
}
```

Using `inject()`:

```javascript
const originalObject = {
  "user": {
    "name": "John",
    "age": 30
  }
};

const flattenedObject = {
  "user.address.street": "123 Main St",
  "user.age": 31
};

const transform = {
  "out": "(: inject(originalObject flattenedObject) :)"
};

const result = DTL.apply({ originalObject, flattenedObject }, transform);
```

Results in:

```json
{
  "user": {
    "name": "John",
    "age": 31,
    "address": {
      "street": "123 Main St"
    }
  }
}
```

### Considerations

- **Overwriting Data**: If a key in the flattened object matches an existing path in the original object, the existing data will be overwritten.
- **Arrays Handling**: Numeric keys in paths lead to the creation of arrays if no existing object is found. Using a '+' symbol appends the value to an existing array.

### Notes

- The `inject()` function does not modify the original object; it returns a new object with the merged data.
- It is ideal for scenarios where you need to add or update data in a deeply nested structure without altering the entire object.
- Care should be taken with key paths to ensure they accurately reflect the intended structure of the resulting object.