Support for multiple Svelte versions?

I’ve been working with a very sizable business app that’s been stuck in Svelte 1 land for… a while.

Because it’s reasonably large, transitioning the whole site in one go would be difficult, but it’s all in one project so I’m trying to work out ways to transition chunks to Svelte 3.

With npm v6.9.0 you can alias different versions of the same library, so you could do:

"dependencies": {
   "svelte1": "npm:svelte@1",
   "svelte3": "npm:svelte@3"
}

However, this plugin puts the Svelte version checking at initialization, e.g. in index.js it has:

const { version } = require('svelte/package.json');
// ...
const major_version = +version[0];
// ...
const { compile, preprocess } = major_version >= 3
	? require('svelte/compiler.js')
	: require('svelte');

Proposal?

Imagine if the version of Svelte could be passed in as an option:

import svelte1 from 'svelte1' // through the magic of alias
import svelte3 from 'svelte3'
import sveltePlugin from 'rollup-plugin-svelte'

export default {
  // ...
  plugins: [
    sveltePlugin({
      svelte: svelte1,
      extensions: [ '.html' ]
    }),
    sveltePlugin({
      svelte: svelte3,
      extensions: [ '.svelte' ]
    }),
    // ...

It looks like the compile and preprocess are only used in the returned transform function, so this could be a non-breaking change:

function loadSvelte() {
	const { version } = require('svelte/package.json');
	const major_version = +version[0];
	return major_version >= 3
		? require('svelte/compiler.js')
		: require('svelte');
}
module.exports = function svelte(options = {}) {
	const { compile, preprocess } = options.svelte || loadSvelte()

I think the general idea is sound, but improvements could probably be made.

[lukeed]

Closed via #124 & will be available in 6.1.0

[tivac]

I built a v2-to-v3 translation library, parts of that night be a useful starting point. We used it for a reasonably large long term port of an existing v2 app to v3.

https://github.com/tivac/svelte-translator

Here’s some details about the port we did using it.

https://twitter.com/tivac/status/1291426619626647557

It actually started life as a v1 app, we ported to v2 using the automated conversion stuff and that was a relatively painless process.

The biggest thing was a simple replacement for this plugin we could use with rollup that was v2 aware, since we didn’t need CSS ha doing it was pretty straightforward.