0.0.64 • Public • Published

    Sapper Site Generator

    a very experimental static site generator overlay on top of Sapper.

    Because Sapper needs fixes to support static export at scale, and moves too slowly for the development of this project, we use a light fork of Sapper ( instead of sapper itself. Hopefully this fork will not be necessary in future, but for now we need these fixes for ssg to work. We aim to keep this fork a superset of sapper as much as possible.

    ssg's Data Fetching Philosophy

    SSG uses a new data fetching paradigm, so understanding this is critical to understanding how to write plugins and how SSG can be parallelizable as well as supports incremental builds. (long term, not exactly currently implemented)

    Most static site generators have a very naive approach to data - pull everything upfront, then use all that upfront data to generate every single page. This makes page generation blocked by the data fetching phase, hence it cannot be parallelized. It also means total agnosticism to data model and every page has to be regenerated every time, hence incremental builds are hard.

    SSG takes the insight that data indexes are cheap, and data slices are expensive. For example, it is cheap to get the filename and date modified info of every file in a directory, or an index of all entries in a CMS. It is more expensive to actually open up, read, and process each file/entry in the CMS. So we split up the data fetch process into a cheap createIndex, and an expensive, but parallelizable, getDataSlice.

    Lastly, the cheap indexes can also serve as a data manifest, which, if saved, can be diffed against prior manifests, so only new/modified data slices need be generated. This gets us incremental builds.

    (again, not currently implemented, but this is the plan)

    How It Works

    the sequence is:

    • start (ssg build or ssg dev)
    • look around the project filesystem for markdown files. this gets put into mainIndex.ssgCoreData. this is meant to help ssg be zero config
    • if ssg.config.js exists, run createIndex and add whatever else you like to mainIndex under whatever keys you want.
    • ssg/Sapper tries to generate a main/index page (eg list of posts)
    • inside any svelte file in src/routes, the Sapper preload function has been modified to have a special this.ssgData function which is a simple helper for pinging a centralized data API route (required if you want to fetch data from ssg.. required for Sapper but eventually we want to get rid of this too)
    • the data route returns the index data, from createIndex. you may specify a key - either ssgCoreData (what this.ssgData assumes by default) or whatever other custom key you want
    • in ssg build, Sapper crawls the index page for links to detail pages (eg individual post) and starts to generate them, in parallel. for ssg dev, only the currently viewed page is generated.
    • each page calls the data route again for data slices
    • the data route calls getDataSlice with an optional key and an optional uid. A uid uniquely identifies the data slice.
    • then the page is generated

    in future we will persist the indexes as manifests, and then diff them between builds so we only run getDataSlice for slices that have changed. for incremental builds.

    Example code

    1. fetching data - you can fetch inside createIndex and getDataSlice.
    // ssg.config.js
    exports.createIndex = async (mainIndex = {}) => {
      console.log('getting intial data')
      mainIndex.MyKey1 = await fetch('whatever/1')
      mainIndex.MyKey2 = await fetch('whatever/2')
      return mainIndex

    this is meant for grabbing indexes cheaply. mainIndex is a POJO that will be use and persisted through the ssg build session, add whatever index you want with a unique key. try not to outright replace mainIndex itself

    1. getDataSlice
    // ssg.config.js
    exports.getDataSlice = async (key, uid) => {
        if (key == 'MyKey1') {
            return fetch(`whatever/1/${uid}`)
        if (key == 'MyKey2') {
            return fetch(`whatever/2/${uid}`)

    as you can see, we are grouping by lifecycle method, and the indexes don't really interact with each other. so we also have a way to group them by key:

    // ssg.config.js
    exports.plugins = {
      MyKey1: {
        createIndex(mainIndex) {
          return fetch(`whatever/1/`)
        getDataSlice(uid) {
          return fetch(`whatever/2/${uid}`)
      MyKey2: {
        createIndex(mainIndex) {
          return fetch(`whatever/2`)
        getDataSlice(uid) {
          return fetch(`whatever/2/${uid}`)

    which also opens up the way for reusable data plugins:

    // could be an npm package
    function myDataPlugin(identifier) {
      return {
        createIndex() {
          return fetch(identifier)
        getDataSlice(uid) {
          return fetch(`${identifier}/${uid}`)
    // ssg.config.js
    exports.plugins = {
      MyKey1: myDataPlugin('whatever/1'),
      MyKey2: myDataPlugin('whatever/2')
    1. adding the ssg data route

    you can copy and paste this exactly or you can run ssg eject and pick the inbuilt scaffold for data route

    1. example index post src/routes/index.svelte
    <script context="module">
      export function preload({ params, query }) {
        return this.ssgData({ key: 'MyKey1' })
          .then(posts => ({ posts }))
          .catch(err => {
            this.error(500, err.message)
      export let posts
        {#each posts as post}
            <a rel="prefetch" href="/{post.slug}">
    1. example detail post src/routes/[slug].svelte
    <script context="module">
      export async function preload({ params, query }) {
        const post = await this.ssgData({ key: 'MyKey1', id: params.slug })
        return { post }
      export let post
      <h1 id="postTitle">{post.title}</h1>
      {@html post.html}

    as you can see, you are expected to know and use Sapper and Svelte conventions.

    You can experiment around with the SSG Demo Repo. You may run into small bugs, please file them.

    Example usage

    Active Codebases you can see this project in use:

    In v0.x we reserve the right to break APIs without warning. Get involved if you need advance warning.


    yarn add ssg

    Svelte and @ssgjs/sapper are included as direct dependencies, however feel free to also install them if you need to.

    To get going, you will then need something in src/routes, usually an index.svelte file to get started. You can scaffold one by running yarn ssg eject and picking sampleIndex.svelte to see an example. This demo repo will also help show how ssg is meant to be used.


    This project needs feedback and maintainers. In fact if you'd like to take it over please be my guest. I don't have time for this. I'm only doing it cause no one else has.

    This is a very nascent project, you'll run into bugs. report them please and also help add tests. If you have feature suggestions please open an issue first to see if it is something we want, before wasting time on a PR.

    Usage and CLI API

    • ssg eject - scaffold out fallback files used by ssg
    • ssg dev - same as sapper dev, runs data pipeline specified in ssg.config.js and watches those files
    • ssg export - same as sapper export, runs data pipeline specified in ssg.config.js and exports the sapper app as a static site.

    Zero Config

    By default, ssg works as a simple zero config layer over sapper. In fact, for the time being, ssg will endeavor to be a sapper superset as far as possible. It uses the programmatic api behind the cli commands, adding some functionality in the @ssgjs/sapper fork of sapper.

    Fallbacks and ssg eject

    ssg makes these Sapper files optional:

    • src/client.js
    • src/server.js
    • src/service-workers.js
    • src/template.html
    • rollup.config.js

    They are located in the ejectableFiles folder.

    However, you can scaffold out these files with the ssg eject command:

    $ yarn ssg eject
    ✔ Pick files to copy out · template.html, client.js
    ✔ A file exists at src/template.html. Are you sure you want to overwrite? (y/N) · false
    ✔ A file exists at src/client.js. Are you sure you want to overwrite? (y/N) · true
    copied /Users/swyx/Work/community/node_modules/ssg/ejectableFiles/client.js to src/client.js

    Generating pages from data

    1. if you need to get data, you will have a src/routes/data/[ssgData].json.js file in your main Sapper project, that looks like this:
    // src/routes/data/[ssgData].json.js`
    const { getDataSlice, getIndex } = require('ssg/readConfig')
    export async function get(req, res) {
      const { ssgData } = req.params
      const splitSlug = ssgData.split('___ssg___')
      const key = splitSlug[0]
      const uid = splitSlug[1]
      const mainIndex = getIndex()
      let data
      // console.log('getting', key, uid)
      if (uid === 'index') {
        data = mainIndex[key]
      } else {
        data = await getDataSlice(key, uid)
      if (typeof data !== 'undefined') {
        res.writeHead(200, { 'Content-Type': 'application/json' })
      } else {
        res.writeHead(404, { 'Content-Type': 'application/json' })
        res.end(JSON.stringify({ message: `Not found` }))

    ⚠️ STOP! the filename is extremely important! doublecheck it is src/routes/data/[ssgData].json.js or expect the above code to break

    You can scaffold this file with ssg eject.

    1. If you need to send data, you should have a ssg.config.js that exports a createIndex (run once) and getDataSlice (run each time) function that provides this data:
    // optional. called repeatedly, can be expensive
    exports.getDataSlice = async (key, uid) => {
      console.log('optional getDataSlice action')
      // we dont really use the key here
      if (key === 'posts') {
        if (uid === 'foo') {
          return { title: 'foo', html: '<div> the foo post </div>' }
        } else {
          return { title: 'bar', html: '<div> the bar post </div>' }
      } else {
        throw new Error('invalid key ' + key )
    exports.createIndex = async (mainIndex = {}) => {
      // do expensive initial fetches and cache them in .ssg/data.json
      mainIndex.index = [{ title: 'foo', slug: 'foo' }, { title: 'bar', slug: 'bar' }]
      return mainIndex
    // optional lifecycle hook
    exports.postExport = async mainIndex => {
      // eg for RSS
      console.log('postExport', mainIndex)

    In your templates, you may now query this data at any time:

    <!-- src/routes/talks/[slug].svelte -->
    <script context="module">
      export async function preload({ params, query }) {
        cosnt key = 'posts'
        const res = await this.ssgData({ key, id: params.slug }) // defaults to key: 'ssgCoreData' and id: 'index'
        if (res.status === 200) {
          return data
        } else {
          this.error(res.status, data.message)
      export let data

    When we drop Sapper we'll likely have a more ergonomic api for this.

    Core Data

    As of v0.45 ssg now also reads all markdown files in the root directory by default. This is inline with 11ty's practice and is configurable by setting a coreDataDirPath string in ssg.config.js:

    // example ssg.config.js
    exports.coreDataOpts = {
      coreDataDirPath: 'content/blog' // defaults to '.'

    ssg dev

    Under the hood, ssg runs sapper dev for you, and watches and reloads it whenever you change your config or contents folder.

    It runs createIndex once and saves that result to a cache, and then you can run getDataSlice anytime you want to query that cache.


    You can also use plugins that have prewritten createIndex and getDataSlice for you:

    // ssg.config.js
    const remark = require('@ssgjs/source-remark')
    const writing = remark({ dirPath: 'content/writing' })
    const speaking = remark({ dirPath: 'content/talks' })
    // optional data plugins. must be object, so we can namespace
    exports.plugins = {


    You can run ssg export to export just like sapper export does. for convenience, I've included a netlify.toml config so you dont have to look it up. Just ssg eject.


    ssg uses debug to log diagnostic messages. Set a Node env variable to enable this logging:

    DEBUG=ssg ssg dev
    # or DEBUG=* ssg export 

    You have a few more degrees of control available incl filtering out messages, look at the debug docs for more ideas.


    npm i ssg

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