Proof-of-concept for transform plugins

example/compile.js

@@ -0,0 +1,10 @@
+const svelte = require("svelte/compiler");
+
+process.stdin.on("data", (buf) => {
+	const result = svelte.compile(buf.toString("utf8"), {
+		generate: "dom",
+		hydratable: true,
+		format: "esm",
+	});
+	console.log(result.js.code);
+});

example/hello.svelte

@@ -0,0 +1 @@
+<h1>hi world!</h1>

example/main.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+
+	"github.com/evanw/esbuild/internal/fs"
+	"github.com/evanw/esbuild/pkg/api"
+)
+
+func main() {
+	result := api.Build(api.BuildOptions{
+		EntryPoints: []string{os.Args[1]},
+		Format:      api.FormatESModule,
+		Bundle:      true,
+		Loaders:     map[string]api.Loader{".svelte": api.LoaderJS},
+		FS:          &SvelteFS{fs.RealFS()},
+	})
+	for _, warn := range result.Warnings {
+		fmt.Println("[WARN] ", warn.Text)
+	}
+	for _, err := range result.Errors {
+		fmt.Println("[ERROR] ", err.Text)
+	}
+	for _, file := range result.OutputFiles {
+		fmt.Println(string(file.Contents))
+	}
+}
+
+// SvelteFS filesystem
+//
+// The idea here is to wrap the existing filesytem
+// in a filesystem that transforms.
+type SvelteFS struct {
+	fs.FS
+}
+
+var _ fs.FS = (*SvelteFS)(nil)
+
+// ReadFile is transforms any .svelte file
+func (fs *SvelteFS) ReadFile(path string) (string, bool) {
+	if filepath.Ext(path) == ".svelte" {
+		code, ok := fs.FS.ReadFile(path)
+		if !ok {
+			return "", ok
+		}
+		// executes ./demo/compile.js
+		// could be amortized by running it when initializing SvelteFS
+		cmd := exec.Command("node", filepath.Join("example", "compile.js"))
+		var stdout bytes.Buffer
+		cmd.Stdout = &stdout
+		cmd.Stderr = os.Stderr
+		cmd.Stdin = bytes.NewBufferString(code)
+		err := cmd.Run()
+		if err != nil {
+			return "", false
+		}
+		// fmt.Println(string(stdout.String()))
+		return stdout.String(), true
+	}
+	return fs.FS.ReadFile(path)
+}