astro/packages/integrations/mdx/src/README.md

# Internal documentation

## rehype-optimize-static

The `rehype-optimize-static` plugin helps optimize the intermediate [`hast`](https://github.com/syntax-tree/hast) when processing MDX, collapsing static subtrees of the `hast` as a `"static string"` in the final JSX output. Here's a "before" and "after" result:

Before:

```jsx
function _createMdxContent() {
  return (
    <>
      <h1>My MDX Content</h1>
      <pre>
        <code class="language-js">
          <span class="token function">console</span>
          <span class="token punctuation">.</span>
          <span class="token function">log</span>
          <span class="token punctuation">(</span>
          <span class="token string">'hello world'</span>
          <span class="token punctuation">)</span>
        </code>
      </pre>
    </>
  );
}
```

After:

```jsx
function _createMdxContent() {
  return (
    <>
      <h1>My MDX Content</h1>
      <pre set:html="<code class=...</code>"></pre>
    </>
  );
}
```

> NOTE: If one of the nodes in `pre` is MDX, the optimization will not be applied to `pre`, but could be applied to the inner MDX node if its children are static.

This results in fewer JSX nodes, less compiled JS output, and less parsed AST, which results in faster Rollup builds and runtime rendering.

To acheive this, we use an algorithm to detect `hast` subtrees that are entirely static (containing no JSX) to be inlined as `set:html` to the root of the subtree.

The next section explains the algorithm, which you can follow along by pairing with the [source code](./rehype-optimize-static.ts). To analyze the `hast`, you can paste the MDX code into https://mdxjs.com/playground.

### How it works

Two variables:

- `allPossibleElements`: A set of subtree roots where we can add a new `set:html` property with its children as value.
- `elementStack`: The stack of elements (that could be subtree roots) while traversing the `hast` (node ancestors).

Flow:

1. Walk the `hast` tree.
2. For each `node` we enter, if the `node` is static (`type` is `element` or `mdxJsxFlowElement`), record in `allPossibleElements` and push to `elementStack`.
    - Q: Why do we record `mdxJsxFlowElement`, it's MDX? <br>
      A: Because we're looking for nodes whose children are static. The node itself doesn't need to be static.
    - Q: Are we sure this is the subtree root node in `allPossibleElements`? <br>
      A: No, but we'll clear that up later in step 3.
3. For each `node` we leave, pop from `elementStack`. If the `node`'s parent is in `allPossibleElements`, we also remove the `node` from `allPossibleElements`.
    - Q: Why do we check for the node's parent? <br>
      A: Checking for the node's parent allows us to identify a subtree root. When we enter a subtree like `C -> D -> E`, we leave in reverse: `E -> D -> C`. When we leave `E`, we see that it's parent `D` exists, so we remove `E`. When we leave `D`, we see `C` exists, so we remove `D`. When we leave `C`, we see that its parent doesn't exist, so we keep `C`, a subtree root.
4. _(Returning to the code written for step 2's `node` enter handling)_ We also need to handle the case where we find non-static elements. If found, we remove all the elements in `elementStack` from `allPossibleElements`. This happens before the code in step 2.
    - Q: Why? <br>
      A: Because if the `node` isn't static, that means all its ancestors (`elementStack`) have non-static children. So, the ancestors couldn't be a subtree root to be optimized anymore.
    - Q: Why before step 2's `node` enter handling? <br>
      A: If we find a non-static `node`, the `node` should still be considered in `allPossibleElements` as its children could be static.
5. Walk done. This leaves us with `allPossibleElements` containing only subtree roots that can be optimized.
6. Add the `set:html` property to the `hast` node, and remove its children.
7. 🎉 The rest of the MDX pipeline will do its thing and generate the desired JSX like above.

### Extra

#### MDX custom components

Astro's MDX implementation supports specifying `export const components` in the MDX file to render some HTML elements as Astro components or framework components. `rehype-optimize-static` also needs to parse this JS to recognize some elements as non-static.

#### Further optimizations

In [How it works](#how-it-works) step 4,

> we remove all the elements in `elementStack` from `allPossibleElements`

We can further optimize this by then also emptying the `elementStack`. This ensures that if we run this same flow for a deeper node in the tree, we don't remove the already-removed nodes from `allPossibleElements`.

While this breaks the concept of `elementStack`, it doesn't matter as the `elementStack` array pop in the "leave" handler (in step 3) would become a no-op.

Example `elementStack` value during walking phase:

```
Enter: A
Enter: A, B
Enter: A, B, C
(Non-static node found): <empty>
Enter: D
Enter: D, E
Leave: D
Leave: <empty>
Leave: <empty>
Leave: <empty>
Leave: <empty>
```