Over onsveltejs/esrap#68 we're working on makingesrap pluggable, so that it can be used to print any AST composed of{ type: string, ... } nodes rather than justestree and its TypeScript extensions. That includes Svelte ASTs.

The main motivation for exposing this is so that we can make it easier to write preprocessors. Historically, Svelte exposed apreprocess API, but it's all strings and duct tape, and it's difficult to integrate preprocessors cleanly with bundlers as we've seen withenhanced-img.

When the preprocessor API was introduced, things looked very different. Preprocessing was necessary to support things like TypeScript and Sass. Today, TypeScript is supported natively, and CSS is sufficiently capable that Sass is little more than a historical curiosity.

In the long term, we'd therefore like to move away from the preprocessor API in favour of providing more robust lower-level utilities. For exampleenhanced-img, which can only be used in a Vite context, really should just be a Vite plugin:

import{parse,print}from'svelte/compiler';import{walk}from'zimmerframe';functiontransform(code){constast=parse(code);consttransformed=walk(ast,null,{RegularElement(node,context){if(node.name!=='enhanced:img')return;// ...}});returnprint(ast);}

There are other potential uses, such as migrations orsv add or having a 'format' button in the playground.

As a side-effect, quality of compiler output will be slightly better in certain cases, such as when encountering comments inside nodes. (Today, we attachleadingComments andtrailingComments to each node, but this is a brittle and not-very-widely-used convention. The newesrap API expects an array ofcomments to be passed instead.)

This functionality already exists insvelte-ast-print (thank you@xeho91!), but having it in core means we can re-use theesrap version that's already installed alongsidesvelte, and will help ensure it stays current with new features.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC:https://github.com/sveltejs/rfcs
• Prefix your PR title withfeat:,fix:,chore:, ordocs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code withinpackages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests withpnpm test and lint the project withpnpm lint