Fix nil pointer panic in FormatOnEnter

internal/format/format_test.go

@@ -1,6 +1,7 @@
 package format_test
 
 import (
+	"context"
 	"strings"
 	"testing"
 
@@ -58,3 +59,62 @@ func TestFormatNoTrailingNewline(t *testing.T) {
 		})
 	}
 }
+
+// Test for panic handling request textDocument/onTypeFormatting (issue #2042)
+// The panic occurs when nodes in a list can be nil, causing deriveActualIndentationFromList
+// to panic when accessing node properties
+func TestFormatOnEnter_NilNodesInList(t *testing.T) {
+	t.Parallel()
+
+	// Test cases that can produce nil nodes in AST lists
+	testCases := []struct {
+		name     string
+		text     string
+		position int // position where enter is pressed
+	}{
+		{
+			name:     "empty file",
+			text:     "",
+			position: 0,
+		},
+		{
+			name:     "simple statement",
+			text:     "const x = 1;",
+			position: 12,
+		},
+		{
+			name:     "incomplete code",
+			text:     "if (",
+			position: 4,
+		},
+		{
+			name:     "malformed syntax",
+			text:     "function f() { return }",
+			position: 21,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			sourceFile := parser.ParseSourceFile(ast.SourceFileParseOptions{
+				FileName: "/test.ts",
+				Path:     "/test.ts",
+			}, tc.text, core.ScriptKindTS)
+
+			ctx := format.WithFormatCodeSettings(context.Background(), &format.FormatCodeSettings{
+				EditorSettings: format.EditorSettings{
+					TabSize:             4,
+					IndentSize:          4,
+					NewLineCharacter:    "\n",
+					ConvertTabsToSpaces: true,
+					IndentStyle:         format.IndentStyleSmart,
+				},
+			}, "\n")
+
+			// This should not panic even with nil nodes in lists
+			edits := format.FormatOnEnter(ctx, sourceFile, tc.position)
+			_ = edits // Just ensuring no panic
+		})
+	}
+}

internal/format/indent.go

@@ -177,13 +177,19 @@ func deriveActualIndentationFromList(list *ast.NodeList, index int, sourceFile *
 	debug.Assert(list != nil && index >= 0 && index < len(list.Nodes))
 
 	node := list.Nodes[index]
+	if node == nil {
+		return -1
+	}
 
 	// walk toward the start of the list starting from current node and check if the line is the same for all items.
 	// if end line for item [i - 1] differs from the start line for item [i] - find column of the first non-whitespace character on the line of item [i]
 
 	line, char := getStartLineAndCharacterForNode(node, sourceFile)
 
 	for i := index; i >= 0; i-- {
+		if list.Nodes[i] == nil {
+			continue
+		}
 		if list.Nodes[i].Kind == ast.KindCommaToken {
 			continue
 		}