Pass adapters to config directly?

[Rich-Harris]

At the moment you do this:

module.exports = {
  kit: {
    adapter: ['@sveltejs/adapter-node', { whatever: 42 }]
  }
};

It might be nicer to do this...

const node = require('@sveltejs/adapter-node');

module.exports = {
  kit: {
    adapter: node({ whatever: 42 })
  }
};

It would make testing and experimentation and debugging easier, and is just generally preferable to passing a string (or a [string, options] array which is a pattern I've always disliked).

Two downsides: it means adapters load eagerly rather than lazily (i.e. they get loaded in dev, even though they don't do anything) and it would mean adapters needed to expose CommonJS rather than ESM, unless we did this...

import node from '@sveltejs/adapter-node';

export default {
  kit: {
    adapter: node({ whatever: 42 });
  }
};

...which we can't at the moment because language tools requires the config to be CommonJS.

[Rich-Harris]

Another benefit of importing the adapters: the options can be typed. Proposed shape:

type Adapter = {
  adapt: (builder: Builder) => void | Promise<void>
};

Then an adapter author can do this (as long as @sveltejs/kit is installed as a dev dependency):

/**
 * @param {{ whatever: number }} options
 * /
export default async function adapter(options) {
  /** @type {import('@sveltejs/kit').Adapter */
  return {
    adapt: builder => {
      // `builder` is typed
    }
  };
}

Note that this is a different signature to the adapters at the moment, which expose a (builder, options) => void function as default. Would be nice to resolve this before the public beta, though @dummdidumm indicated that it would tricky to have an ESM config file (#349 (comment)) so we might be stuck shipping CJS for the time being.