# Data Model

Slate is based on an immutable data model that closely resembles the DOM. When you start using Slate, one of the most important things to do is familiarize yourself with this data model. This guide will help you do just that!

## Mirror the DOM

One of the main principles of Slate is that it tries to mirror the native DOM API's as much as possible.

If you think about it, this makes sense. Slate is kind of like a nicer implementation of `contenteditable`, which itself is built with the DOM. And people use the DOM to represent documents with rich-text-like structures all the time. Mirroring the DOM helps make the library familiar for new users, and it lets us reuse battle-tested patterns without having to reinvent them ourselves.

Because it mirrors the DOM, Slate's data model features a [`Document`](../reference/slate/document.md) with [`Block`](../reference/slate/block.md), [`Inline`](../reference/slate/inline.md) and [`Text`](../reference/slate/text.md) nodes. You can reference parts of the document with a [`Range`](../reference/slate/range.md). And there is a special range-like object called a [`Selection`](../reference/slate/selection.md) that represents the user's current cursor selection.

## Immutable Objects

Slate's data model is built out of [`Immutable.js`](https://facebook.github.io/immutable-js/) objects. This allows us to make rendering much more performant, and it ensures that we don't end up with hard to track down bugs due to accidentally modifying objects in-place.

Specifically, Slate's models are [`Immutable.Record`](https://facebook.github.io/immutable-js/docs/#/Record) objects, which makes them very similar to Javascript objects for retrieiving values:

```js
const block = Block.create({ type: 'paragraph' })

block.object // "block"
block.type // "paragraph"
```

But for updating values, you'll need to use the [`Immutable.Record` API](https://facebook.github.io/immutable-js/docs/#/Record/set).

Collections of Slate objects are represented as immutable `Lists`, `Sets`, `Stacks`, etc, which means we get nice support for expressive methods like `filter`, `includes`, `take`, `skip`, `rest` and `last`.

If you haven't used Immutable.js before, there is definitely a learning curve. Before you dive into Slate, you should check out the [Immutable.js docs](https://facebook.github.io/immutable-js/docs/#/). Once you get the hang of it, it won't slow you down at all, but it will take a few days to get used to, and you might write things suboptimally at first.

## The "Value"

The top-level object in Slate—the object encapsulating the entire value of an Slate editor—is called a [`Value`](../reference/slate/value.md).

It is made up of a document filled with content, and a selection representing the user's current cursor selection. It also has a history, to keep track of changes, and a few other more advanced properties like `decorations` and `data`.

> 📋 For more info, check out the [`Value` reference](../reference/slate/value.md).

## Documents and Nodes

Slate documents are nested and recursive. This means that a document has block nodes, and those block nodes can have child nodes—all the way down. This lets you model more complex nested behaviors like tables and figures with captions.

Unlike the DOM though, Slate enforces a few more restrictions on its documents. This reduces the complexity involved in manipulating them and prevents "impossible" situations from arising. These restrictions are:

* **Documents can only contain block nodes as direct children.** This restriction mirrors how rich-text editors work, with the top-most elements being blocks that can be split when pressing <kbd>enter</kbd>.

* **Blocks can only contain either other block nodes, or inlines and text nodes.** This is another "sane" restriction that allows you to avoid lots of boilerplate `if` statements in your code. Blocks either wrap other blocks, or contain actual content.

* **Inlines can only contain inline or text nodes.** This one is also for sanity and avoiding boilerplate. Once you've descended into an "inline" context, you can't have block nodes inside them.

* **Text nodes can't be adjacent to other text nodes.** Any two adjacent text nodes will automatically be merged into one. This prevents ambiguous cases where a cursor could be at the end of one text node or at the start of the next. However, you can have an inline node surrounded by two texts.

* **Blocks and inlines must always contain at least one text node.** This is to ensure that the user's cursor can always "enter" the nodes and to make sure that ranges can be created referencing them.

Slate enforces all of these restrictions for you automatically. Any time you [perform changes](./changes.md) to the document, Slate will check if the document is invalid, and if so, it will return it to a "normalized" value.

> 🙃 Fun fact: "normalizing" is actually based on the DOM's [`Node.normalize()`](https://developer.mozilla.org/en-US/docs/Web/API/Node/normalize)!

In addition to documents, blocks and inlines, Slate introduces one other type of markup that the DOM doesn't have natively, the [`Mark`](../reference/slate/mark.md).

## Marks

Marks are how Slate represents formatting data that is attached to the characters in the text itself—things like **bold**, _italic_, `code`, or even more complex formatting like comments.

Although you can change styling based on either inlines or marks, marks differ from inlines in that they don't affect the structure of the nodes in the document, they simply attach themselves to the characters.

This makes marks easier to reason about and easier to manipulate. Because inlines involve editing the document's structure, you have to worry about things like splitting any existing nodes, what their order in the hierarchy is, etc. Marks on the other hand can be applied to characters no matter how the characters are nested in the document. If you can express it as a `Range`, you can add marks to it.

But this also has implications on how marks are rendered. When marks are rendered, the characters are grouped into "leaves" of text that each contain the same set of marks applied to them. But you cannot guarantee how a set of marks will be ordered.

This is actually similar to the DOM, where this is invalid:

```html
<em>t<strong>e</em>x</strong>t
```

Because the elements don't properly close themselves. Instead you have to write it like this:

```html
<em>t</em><strong><em>e</em>x</strong>t
```

And if you happened to add another overlapping section of `<strike>` to that text, you might have to rearrange the closing tags again. Rendering marks in Slate is similar—you can't guarantee that even though a word has one mark applied that that mark will be contiguous, because it depends on how it overlaps with other marks.

That all sounds pretty complex, but you don't have to think about it much, as long as you use marks and inlines for their intended purposes...

* Marks represent **unordered**, character-level formatting.
* Inlines represent **contiguous**, semantic elements in the document.

## Ranges, Points and "The Selection"

Just like in the DOM, you can reference a part of the document using a `Range`. And there's one special range that Slate keeps track of that refers to the user's current cursor selection, called the `Selection`.

Ranges are defined by two `Point`s, an "anchor" point and a "focus" point. The anchor is where the range starts, and the focus is where it ends. And each point is a combination of a "path" or "key" referencing a specific node, and an "offset". This ends up looking like this:

```js
const range = Range.create({
  anchor: {
    key: 'node-a',
    path: [0, 2, 1],
    offset: 0,
  },
  focus: {
    key: 'node-b',
    path: [0, 3, 2],
    offset: 4,
  },
})
```

The more readable `node-a` name is just pseudocode, because Slate uses auto-incrementing numerical strings by default—`'1', '2', '3', ...` But the important part is that every node has a unique `key` property, and a range can reference nodes by their keys.

The terms "anchor" and "focus" are borrowed from the DOM API. The anchor point isn't always _before_ the focus point in the document. Just like in the DOM, it depends on whether the range is backwards or forwards.

Here's how MDN explains it:

> A user may make a selection from left to right (in document order) or right to left (reverse of document order). The anchor is where the user began the selection and the focus is where the user ends the selection. If you make a selection with a desktop mouse, the anchor is placed where you pressed the mouse button and the focus is placed where you released the mouse button. Anchor and focus should not be confused with the start and end positions of a selection, since anchor can be placed before the focus or vice versa, depending on the direction you made your selection.
> — [`Selection`, MDN](https://developer.mozilla.org/en-US/docs/Web/API/Selection)

To make dealing with ranges easier though, `Range` objects also provide `start` and `end` point properties that take whether the range is forward or backward into account. The `start.key` and `start.path` will always be before the `end.key` and `end.path` in the document.

These `start` and `end` points are what most of your logic will be based on, since you rarely care which side of the selection is "extendable".

One important thing to note is that the anchor and focus points of ranges **always reference the "leaf-most" text nodes** in a document. They never reference blocks or inlines, always their child text nodes. This is different than in the DOM API, but it makes dealing with ranges a _lot_ easier because there are less edge cases to handle.

> 📋 For more info, check out the [`Range` reference](../reference/slate/range.md).

The `Selection` model contains slightly more information than the simple `Range` model, because it needs to keep track of "focus" and "marks" for the user. For example:

```js
const selection = Selection.create({
  anchor: {
    key: 'node-a',
    path: [0, 2, 1],
    offset: 0,
  },
  focus: {
    key: 'node-b',
    path: [0, 3, 2],
    offset: 4,
  },
  isFocused: true,
  marks: [{ type: 'bold' }],
})
```

However, keeping the `key` and `path` of ranges or selections in sync yourself is tedious. Instead, you can create selections using either and have the other automatically be inferred by the document. To do that, you use the `createRange` and `createSelection` methods:

```js
const selection = document.createSelection({
  anchor: {
    key: 'node-a',
    offset: 0,
  },
  focus: {
    key: 'node-b',
    offset: 4,
  },
  isFocused: true,
  marks: [{ type: 'bold' }],
})
```

The resulting `selection` will have a both the `key` and `path` set for its points, with the `key` being used to look up the `path` in the document.