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

src/Controls/src/Core/WebView/WebView.cs

@@ -4,11 +4,11 @@
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Net;
-using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using Microsoft.Maui.Controls.Internals;
 
 using Microsoft.Maui.Devices;
+using Microsoft.Maui.Handlers;
 
 namespace Microsoft.Maui.Controls
 {
@@ -125,7 +125,7 @@ public async Task<string> EvaluateJavaScriptAsync(string script)
 			// Make all the platforms mimic Android's implementation, which is by far the most complete.
 			if (DeviceInfo.Platform != DevicePlatform.Android)
 			{
-				script = EscapeJsString(script);
+				script = WebViewHelper.EscapeJsString(script);
 
 				if (DeviceInfo.Platform != DevicePlatform.WinUI)
 				{
@@ -290,43 +290,6 @@ public IPlatformElementConfiguration<T, WebView> On<T>() where T : IConfigPlatfo
 			return _platformConfigurationRegistry.Value.On<T>();
 		}
 
-		private static string EscapeJsString(string js)
-		{
-			if (js == null)
-				return null;
-
-			if (js.IndexOf("'", StringComparison.Ordinal) == -1)
-				return js;
-
-			//get every quote in the string along with all the backslashes preceding it
-			var singleQuotes = Regex.Matches(js, @"(\\*?)'");
-
-			var uniqueMatches = new List<string>();
-
-			for (var i = 0; i < singleQuotes.Count; i++)
-			{
-				var matchedString = singleQuotes[i].Value;
-				if (!uniqueMatches.Contains(matchedString))
-				{
-					uniqueMatches.Add(matchedString);
-				}
-			}
-
-			uniqueMatches.Sort((x, y) => y.Length.CompareTo(x.Length));
-
-			//escape all quotes from the script as well as add additional escaping to all quotes that were already escaped
-			for (var i = 0; i < uniqueMatches.Count; i++)
-			{
-				var match = uniqueMatches[i];
-				var numberOfBackslashes = match.Length - 1;
-				var slashesToAdd = (numberOfBackslashes * 2) + 1;
-				var replacementStr = "'".PadLeft(slashesToAdd + 1, '\\');
-				js = Regex.Replace(js, @"(?<=[^\\])" + Regex.Escape(match), replacementStr);
-			}
-
-			return js;
-		}
-
 		/// <inheritdoc/>
 		IWebViewSource IWebView.Source => Source;
 

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

@@ -0,0 +1,153 @@
+using Xunit;
+using Microsoft.Maui.Handlers;
+
+namespace Microsoft.Maui.Controls.Core.UnitTests;
+
+public class WebViewHelperTests
+{
+	[Fact]
+	public void EscapeJsString_NullInput_ReturnsNull()
+	{
+		const string input = null;
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Null(result);
+	}
+
+	[Fact]
+	public void EscapeJsString_NoSingleQuote_ReturnsSameString()
+	{
+		const string input = """console.log("Hello, world!");""";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(input, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_UnescapedQuote_EscapesCorrectly()
+	{
+		// Each unescaped single quote should be preceded by one backslash.
+		const string input = """console.log('Hello, world!');""";
+		// Expected: each occurrence of "'" becomes "\'"
+		const string expected = """console.log(\'Hello, world!\');""";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_AlreadyEscapedQuote_EscapesFurther()
+	{
+		const string input = """var str = 'Don\'t do that';""";
+		const string expected = """var str = \'Don\\\'t do that\';""";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_MultipleLinesAndMixedQuotes()
+	{
+		const string input = """
+			function test() {
+				console.log('Test "string" with a single quote');
+				var example = 'It\\'s tricky!';
+			}
+			""";
+		const string expected = """
+			function test() {
+				console.log(\'Test "string" with a single quote\');
+				var example = \'It\\\\\'s tricky!\';
+			}
+			""";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_MultipleBackslashesBeforeQuote()
+	{
+		const string input = @"var tricky = 'Backslash: \\\' tricky!';";
+		const string expected = @"var tricky = \'Backslash: \\\\\\\' tricky!\';";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_QuoteAtBeginning()
+	{
+		const string input = @"'Start with quote";
+		const string expected = @"\'Start with quote";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_QuoteAtEnd()
+	{
+		const string input = @"Ends with a quote'";
+		const string expected = @"Ends with a quote\'";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_OnlyQuote()
+	{
+		const string input = @"'";
+		const string expected = @"\'";
+		var result = WebViewHelper.EscapeJsString(input);
+		Assert.Equal(expected, result);
+	}
+
+	[Fact]
+	public void EscapeJsString_RepeatedEscapedQuotes()
+	{
+		const string input = @"'Quote' and again \'Quote\'";
+		const string expected = @"\'Quote\' and again \\\'Quote\\\'";
+		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/HybridWebView/HybridWebViewHandler.cs

@@ -335,7 +335,7 @@ public static async void MapEvaluateJavaScriptAsync(IHybridWebViewHandler handle
 			// Make all the platforms mimic Android's implementation, which is by far the most complete.
 			if (!OperatingSystem.IsAndroid())
 			{
-				script = EscapeJsString(script);
+				script = WebViewHelper.EscapeJsString(script);
 
 				if (!OperatingSystem.IsWindows())
 				{
@@ -430,47 +430,6 @@ await handler.InvokeAsync(nameof(IHybridWebView.EvaluateJavaScriptAsync),
 			}
 		}
 
-
-#if PLATFORM && !TIZEN
-		// Copied from WebView.cs
-		internal static string? EscapeJsString(string js)
-		{
-			if (js == null)
-				return null;
-
-			if (!js.Contains('\'', StringComparison.Ordinal))
-				return js;
-
-			//get every quote in the string along with all the backslashes preceding it
-			var singleQuotes = Regex.Matches(js, @"(\\*?)'");
-
-			var uniqueMatches = new List<string>();
-
-			for (var i = 0; i < singleQuotes.Count; i++)
-			{
-				var matchedString = singleQuotes[i].Value;
-				if (!uniqueMatches.Contains(matchedString))
-				{
-					uniqueMatches.Add(matchedString);
-				}
-			}
-
-			uniqueMatches.Sort((x, y) => y.Length.CompareTo(x.Length));
-
-			//escape all quotes from the script as well as add additional escaping to all quotes that were already escaped
-			for (var i = 0; i < uniqueMatches.Count; i++)
-			{
-				var match = uniqueMatches[i];
-				var numberOfBackslashes = match.Length - 1;
-				var slashesToAdd = (numberOfBackslashes * 2) + 1;
-				var replacementStr = "'".PadLeft(slashesToAdd + 1, '\\');
-				js = Regex.Replace(js, @"(?<=[^\\])" + Regex.Escape(match), replacementStr);
-			}
-
-			return js;
-		}
-#endif
-
 		internal static async Task<string?> GetAssetContentAsync(string assetPath)
 		{
 			using var stream = await GetAssetStreamAsync(assetPath);

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

@@ -0,0 +1,77 @@
+namespace Microsoft.Maui.Handlers;
+
+using System;
+using System.Text.RegularExpressions;
+
+internal static partial class WebViewHelper
+{
+	const string NewlineMarker = "##NL##";
+
+	internal static string? EscapeJsString(string js)
+	{
+		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
+		js = Regex.Replace(js, @"\\n", NewlineMarker);
+
+		// 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))
+#else
+		if (js.IndexOf('\'') == -1)
+#endif
+			return js;
+
+		return EscapeJsStringRegex().Replace(js, m =>
+		{
+			int count = m.Groups[1].Value.Length;
+			// Replace with doubled backslashes plus one extra backslash, then the quote.
+			return new string('\\', (count * 2) + 1) + "'";
+		});
+	}
+
+#if NET6_0_OR_GREATER
+	[GeneratedRegex(@"(\\*)'")]
+	private static partial Regex EscapeJsStringRegex();
+#else
+	static Regex? EscapeJsStringRegexCached;
+	private static Regex EscapeJsStringRegex() =>
+		EscapeJsStringRegexCached ??= new Regex(@"(\\*)'", RegexOptions.Compiled | RegexOptions.CultureInvariant);
+#endif
+}