Fix for WebView.EvaluateJavaScriptAsync fails to execute JavaScript containing newline characters

src/Controls/tests/Core.UnitTests/WebViewHelperTests.cs

@@ -104,4 +104,50 @@ public void EscapeJsString_RepeatedEscapedQuotes()
 		var result = WebViewHelper.EscapeJsString(input);
 		Assert.Equal(expected, result);
 	}
+
+	[Fact]
+	public void EscapeJsString_SimpleJavaScriptWithNewlines()
+	{
+		const string input = "var x = 5;\r\n" +
+							"var y = 10;\r" +
+							"var z = x + y;\n";
+
+		const string expected = "var x = 5;\\nvar y = 10;\\nvar z = x + y;\\n";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_TemplateLiterals()
+	{
+		const string input = @"let poll = `Is .NET MAUI cool?
+- Yes!
+- Yes!
+- Yes!
+Wow, so it is!
+`
+console.log(poll);";
+
+		const string expected = "let poll = \\`Is .NET MAUI cool?\\n- Yes!\\n- Yes!\\n- Yes!\\n" +
+			"Wow, so it is!\\n\\`\\nconsole.log(poll);";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_BackslashContinuations()
+	{
+		const string input = @"let poll = 'Is .NET MAUI cool? \n\
+- Yes! \n\
+- Yes! \n\
+- Yes! \n\
+Wow, so it is! \n\
+'
+console.log(poll);";
+
+		const string expected = "let poll = \\'Is .NET MAUI cool? \\\\n- Yes! \\\\n- Yes! \\\\n" +
+			"- Yes! \\\\nWow, so it is! \\\\n\\'\\nconsole.log(poll);";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
 }

src/Core/src/Handlers/WebView/WebViewHelper.cs

@@ -5,10 +5,54 @@
 
 internal static partial class WebViewHelper
 {
+	const string NewlineMarker = "##NL##";
+
 	internal static string? EscapeJsString(string js)
 	{
-		if (js == null)
-			return null;
+		if (string.IsNullOrEmpty(js))
+			return js;
+
+		// Normalize line endings
+		js = Regex.Replace(js, @"\r\n|\r", "\n");
+
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+		bool hasBacktick = js.Contains('`', StringComparison.Ordinal);
+#else
+		bool hasBacktick = js.IndexOf('`') != -1;
+#endif
+
+		// Escape sequence marker
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+		js = js.Replace("\\n", NewlineMarker, StringComparison.Ordinal);
+#else
+		js = js.Replace("\\n", NewlineMarker);
+#endif
+		// Escape backticks if present
+		if (hasBacktick)
+		{
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+			js = js.Replace("`", "\\`", StringComparison.Ordinal);
+#else
+			js = js.Replace("`", "\\`");
+#endif
+		}
+
+		// Remove backslash-newline continuation
+		js = Regex.Replace(js, @"\\[ \t]*\n", string.Empty);
+
+		// Replace literal newlines with \n
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+		js = js.Replace("\n", "\\n", StringComparison.Ordinal);
+#else
+		js = js.Replace("\n", "\\n");
+#endif
+
+		// Restore original escape sequences
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+		js = js.Replace(NewlineMarker, "\\\\n", StringComparison.Ordinal);
+#else
+		js = js.Replace(NewlineMarker, "\\\\n");
+#endif
 
 #if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
 		if (!js.Contains('\'', StringComparison.Ordinal))