Add js web stuff to landing page + documentation

go/jsbundler/import_check.go

@@ -0,0 +1,87 @@
+package jsbundler
+
+// esbuild silently resolves a `.js` import to a sibling `.ts`/`.tsx`/`.jsx`,
+// which lets misnamed specifiers slip through.  This check catches them up
+// front so every import path names the file that actually exists on disk.
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+type importViolation struct {
+	file   string
+	line   int
+	spec   string // the written specifier, e.g. "./Icons.js"
+	actual string // the corrected specifier, e.g. "./Icons.ts"
+}
+
+// reImportSpec matches a quoted relative module specifier ending in ".js".
+var reImportSpec = regexp.MustCompile(`(["'])((?:\.\.?/)[^"']*?\.js)["']`)
+
+// tsExtensions are the non-.js source extensions a .js specifier may
+// actually resolve to, in esbuild's resolution order.
+var tsExtensions = []string{".ts", ".tsx", ".jsx"}
+
+// checkImportExtensions scans every JS/TS source under frontend/src for
+// relative import specifiers written with a ".js" extension whose literal
+// target does not exist but a sibling ".ts"/".tsx"/".jsx" does. esbuild
+// resolves these transparently, so they bundle fine — but the import path
+// lies about the file it points to. Specifiers that resolve to a real ".js"
+// file, and dangling specifiers with no sibling at all, are left for esbuild.
+func checkImportExtensions() []importViolation {
+	var violations []importViolation
+	srcRoot := filepath.Join(frontendDir, "src")
+
+	filepath.WalkDir(srcRoot, func(path string, d os.DirEntry, err error) error {
+		if err != nil || d.IsDir() {
+			return err
+		}
+		switch filepath.Ext(path) {
+		case ".js", ".ts", ".tsx", ".jsx":
+		default:
+			return nil
+		}
+
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			return err
+		}
+		dir := filepath.Dir(path)
+
+		for i, line := range strings.Split(string(raw), "\n") {
+			for _, m := range reImportSpec.FindAllStringSubmatch(line, -1) {
+				spec := m[2]
+				if _, statErr := os.Stat(filepath.Join(dir, spec)); statErr == nil {
+					continue // resolves to a real .js file — fine
+				}
+				base := strings.TrimSuffix(spec, ".js")
+				for _, ext := range tsExtensions {
+					if _, statErr := os.Stat(filepath.Join(dir, base+ext)); statErr == nil {
+						violations = append(violations, importViolation{
+							file:   path,
+							line:   i + 1,
+							spec:   spec,
+							actual: base + ext,
+						})
+						break
+					}
+				}
+			}
+		}
+		return nil
+	})
+
+	return violations
+}
+
+func reportImportViolations(violations []importViolation) {
+	fmt.Fprintf(os.Stderr, "\nImport extension check failed: %d import(s) use a .js extension for a non-.js file.\n", len(violations))
+	for _, v := range violations {
+		fmt.Fprintf(os.Stderr, "  %s:%d: %q should be %q\n", v.file, v.line, v.spec, v.actual)
+	}
+	fmt.Fprintln(os.Stderr, "Rename each specifier to match the file's real extension.")
+}