Build a Transformer

myTransformer.ts:

import type {
    MdxPreprocessOptionsInput,
    MdxPreprocessOptionsOutput,
} from "mdx-svelte"
import type { MarkupPreprocessor } from "svelte/compiler"

type MyTransformerOptions = {}

export const myTransformer = ((
    // Markdown files (absolute) path and content.
    markup: Parameters<MarkupPreprocessor>[0],

    // The config that is passed to the `mdxPreprocess()`.
    mdxPreprocessOptions: MdxPreprocessOptionsOutput,

    // Your transformer options.
    transformerOptions?: MyTransformerOptions,
) => {
    // ...
}) satisfies MdxPreprocessOptionsInput["onTransform"]

svelte.config.js:

import { myTransformer } from "$lib/myTransformer"

mdxPreprocess({
    onTransform: (markup, options) => {
        return myTransformer(markup, options, {
            // ...
        })
    },
})