fix/class:list directive was not properly merging with the class attribute

.changeset/ten-melons-return.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/compiler": patch
+---
+
+Resolves an issue where the `class:list` directive was not correctly merging with the class attribute.

internal/transform/transform.go

@@ -625,21 +625,35 @@ func walk(doc *astro.Node, cb func(*astro.Node)) {
 func mergeClassList(doc *astro.Node, n *astro.Node, opts *TransformOptions) {
 	var classListAttrValue string
 	var classListAttrIndex int = -1
+
+	var classAttrType astro.AttributeType
 	var classAttrValue string
 	var classAttrIndex int = -1
-	for i, a := range n.Attr {
-		if a.Key == "class:list" {
-			classListAttrValue = a.Val
+
+	for i, attr := range n.Attr {
+		if attr.Key == "class:list" {
+			classListAttrValue = attr.Val
 			classListAttrIndex = i
 		}
-		if a.Key == "class" {
-			classAttrValue = a.Val
+		if attr.Key == "class" {
+			classAttrType = attr.Type
+			classAttrValue = attr.Val
 			classAttrIndex = i
 		}
 	}
+
+	// Check if both `class:list` and `class` attributes are present
 	if classListAttrIndex >= 0 && classAttrIndex >= 0 {
-		// we append the prepend the value of class to class:list
-		n.Attr[classListAttrIndex].Val = fmt.Sprintf("['%s', %s]", classAttrValue, classListAttrValue)
+		// Merge the `class` attribute value into `class:list`
+		if classAttrType == astro.ExpressionAttribute {
+			// If the `class` attribute is an expression, include it directly without surrounding quotes.
+			// This respects the fact that expressions are evaluated dynamically and should not be treated as strings.
+			n.Attr[classListAttrIndex].Val = fmt.Sprintf("[%s, %s]", classAttrValue, classListAttrValue)
+		} else {
+			// If the `class` attribute is a static string, wrap it in quotes.
+			// This ensures that static class names are treated as string values within the list.
+			n.Attr[classListAttrIndex].Val = fmt.Sprintf("['%s', %s]", classAttrValue, classListAttrValue)
+		}
 		// Now that the value of `class` is carried by `class:list`, we can remove the `class` node from the AST.
 		// Doing so will allow us to generate valid HTML at runtime
 		n.Attr = remove(n.Attr, classAttrIndex)

internal/transform/transform_test.go

@@ -234,22 +234,19 @@ func TestFullTransform(t *testing.T) {
 		want   string
 	}{
 		{
-			name: "top-level component with leading style",
-			source: `<style>:root{}</style><Component><h1>Hello world</h1></Component>
-			`,
-			want: `<Component><h1>Hello world</h1></Component>`,
+			name:   "top-level component with leading style",
+			source: `<style>:root{}</style><Component><h1>Hello world</h1></Component>`,
+			want:   `<Component><h1>Hello world</h1></Component>`,
 		},
 		{
-			name: "top-level component with leading style body",
-			source: `<style>:root{}</style><Component><div><h1>Hello world</h1></div></Component>
-			`,
-			want: `<Component><div><h1>Hello world</h1></div></Component>`,
+			name:   "top-level component with leading style body",
+			source: `<style>:root{}</style><Component><div><h1>Hello world</h1></div></Component>`,
+			want:   `<Component><div><h1>Hello world</h1></div></Component>`,
 		},
 		{
-			name: "top-level component with trailing style",
-			source: `<Component><h1>Hello world</h1></Component><style>:root{}</style>
-			`,
-			want: `<Component><h1>Hello world</h1></Component>`,
+			name:   "top-level component with trailing style",
+			source: `<Component><h1>Hello world</h1></Component><style>:root{}</style>`,
+			want:   `<Component><h1>Hello world</h1></Component>`,
 		},
 		{
 			name:   "Component before html I",
@@ -287,15 +284,9 @@ func TestFullTransform(t *testing.T) {
 			want:   `<A><div><B></B></div></A>`,
 		},
 		{
-			name: "does not remove trailing siblings",
-			source: `<title>Title</title>
-<span />
-<Component />
-<span />`,
-			want: `<title>Title</title>
-<span></span>
-<Component></Component>
-<span></span>`,
+			name:   "does not remove trailing siblings",
+			source: `<title>Title</title><span /><Component /><span />`,
+			want:   `<title>Title</title><span></span><Component></Component><span></span>`,
 		},
 	}
 	var b strings.Builder
@@ -532,3 +523,69 @@ func TestAnnotation(t *testing.T) {
 		})
 	}
 }
+
+func TestClassAndClassListMerging(t *testing.T) {
+	tests := []struct {
+		name   string
+		source string
+		want   string
+	}{
+		{
+			name:   "Single class attribute",
+			source: `<div class="astro-xxxxxx" />`,
+			want:   `<div class="astro-xxxxxx"></div>`,
+		},
+		{
+			name:   "Class attribute with parameter",
+			source: "<div class={`astro-xxxxxx ${astro}`} />",
+			want:   "<div class={`astro-xxxxxx ${astro}`}></div>",
+		},
+		{
+			name:   "Single class:list attribute",
+			source: `<div class:list={"astro-xxxxxx"} />`,
+			want:   `<div class:list={"astro-xxxxxx"}></div>`,
+		},
+		{
+			name:   "Merge class with empty class:list (double quotes)",
+			source: `<div class="astro-xxxxxx" class:list={} />`,
+			want:   `<div class:list={['astro-xxxxxx', ]}></div>`,
+		},
+		{
+			name:   "Merge class with empty class:list (single quotes)",
+			source: `<div class='astro-xxxxxx' class:list={} />`,
+			want:   `<div class:list={['astro-xxxxxx', ]}></div>`,
+		},
+		{
+			name:   "Merge class and class:list attributes (string)",
+			source: `<div class="astro-xxxxxx" class:list={"astro-yyyyyy"} />`,
+			want:   `<div class:list={['astro-xxxxxx', "astro-yyyyyy"]}></div>`,
+		},
+		{
+			name:   "Merge class and class:list attributes (expression)",
+			source: `<div class={"astro-xxxxxx"} class:list={"astro-yyyyyy"} />`,
+			want:   `<div class:list={["astro-xxxxxx", "astro-yyyyyy"]}></div>`,
+		},
+		{
+			name:   "Merge Class and Class List Attributes (concatenation)",
+			source: `<div class={"astro-xxxxxx" + name} class:list={"astro-yyyyyy"} />`,
+			want:   `<div class:list={["astro-xxxxxx" + name, "astro-yyyyyy"]}></div>`,
+		},
+	}
+
+	var b strings.Builder
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			b.Reset()
+			doc, err := astro.Parse(strings.NewReader(tt.source))
+			if err != nil {
+				t.Error(err)
+			}
+			Transform(doc, TransformOptions{}, handler.NewHandler(tt.source, "/test.astro"))
+			astro.PrintToSource(&b, doc.LastChild.FirstChild.NextSibling.FirstChild)
+			got := b.String()
+			if tt.want != got {
+				t.Errorf("\nFAIL: %s\n  want: %s\n  got:  %s", tt.name, tt.want, got)
+			}
+		})
+	}
+}