my-loader.js

const simpleGit = require('simple-git/promise')()
const path = require('path')
const { DateTime } = require('luxon')
module.exports = async function (source, map, meta) {
    var callback = this.async()

    let info = await simpleGit.log([this.resourcePath])
    let createdAt = info.all.length
        ? new Date(info.all[info.all.length - 1].date).getTime()
        : null

    let updatedAt = info.all.length ? new Date(info.all[0].date).getTime() : null

    // we are assuming "content" will be here. if we made this more
    // generic we'd allow that to be customized
    // path.sep = /home/will/projects/williamhoyle.ca/content/blog/2020/filename.md

    let parts = this.resourcePath.split('content')

    let href = parts.pop()
    // remove extension
    href = href
        .split('.')
        .slice(0, -1)
        .join('.')

    // enrich the yaml frontmatter before it goes through the loader
    // then, we'll be able to access this info inside our templates
    source = source.replace(
        '---',
        `---
updatedAt: ${updatedAt && DateTime.fromMillis(updatedAt).toFormat('yyyy LLLL d')}
createdAt: ${createdAt && DateTime.fromMillis(createdAt).toFormat('yyyy LLLL d')}
jsCreatedAt: ${createdAt}
jsUpdatedAt: ${updatedAt}
href: ${href}
`
    )

    source = source.replace('[[date]]', `<p class="text-gray-600">${createdAt && DateTime.fromMillis(createdAt).toFormat('yyyy LLLL d')}</p>`)

    callback(null, source, map, meta)
}