Fix for WebView.EvaluateJavaScriptAsync fails to execute JavaScript containing newline characters #29126

praveenkumarkarunanithi · 2025-04-22T11:45:43Z

Root Cause

The EvaluateJavaScriptAsync method did not account for multi-line JavaScript strings containing newline characters (\r\n, \r, \n). These newline characters caused script execution issues on platforms like Windows (WebView2) and iOS (WKWebView), either silently failing to execute or throwing runtime exceptions due to unprocessed newline characters in the script string.

Description of Change

The JavaScript string is now normalized by replacing all newline characters (\r\n, \r, \n) with a space before execution. This ensures consistent behavior and compatibility across platforms.

Issues Fixed

Fixes #11905

Tested the behaviour in the following platforms

Android
Windows
iOS
Mac

Screenshots

Before Issue Fix	After Issue Fix

dotnet-policy-service · 2025-04-22T11:46:00Z

Hey there @@praveenkumarkarunanithi! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

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

MartyIX · 2025-04-23T08:20:55Z

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

+		public async Task TestEscapeJsStringHandlesNewlines()
+		{
+			string scriptWithNewlines = "var x = 5;\r\n" +
+									"var y = 10;\r" +


What does happen when there is javascript code: var myString = "<NEWLINE HERE>"; console.log(myString);? It would be nice to have such a test.

@MartyIX , Our fix replaces literal newline characters with spaces to ensure JavaScript executes properly across all platforms. JavaScript string escape sequences (like \n) are not affected since they're represented differently in code:
string jsWithEscapeSequence = "var myString = '\\n';";
since standard practice is to use \n escape sequences to represent newlines in strings,most real-world code won't be affected.

My point I wanted to make was simply: Your PR can modify original behavior of user's JavaScript code. I believe it should be mentioned in the PR description. If it is acceptable or not, is not up to me. I just want to make sure it's explicitly acknowledged.

btw: #29139 was merged to the inflight branch yesterday. Would it make sense to test against that branch if your workaround is still needed?

I'm basically very interested what causes that on Windows you observe "silent failure". Do you possibly know where the limitation come from? Or what is buggy?

mattleibow

This PR should land after #27528

But, as @MartyIX says, newlines may have meaning in the context of JS. For example, they use backtics to represent the multi line strings:

let poll = `Is .NET MAUI cool?
- Yes!
- Yes!
- Yes!
Wow, so it is!
`
console.log(poll);

as well as backslashes:

let poll = 'Is .NET MAUI cool? \n\
- Yes! \n\
- Yes! \n\
- Yes! \n\
Wow, so it is! \n\
'
console.log(poll);

We need to add tets for these as well.

praveenkumarkarunanithi · 2025-07-02T13:13:00Z

This PR should land after #27528

But, as @MartyIX says, newlines may have meaning in the context of JS. For example, they use backtics to represent the multi line strings:
let poll = `Is .NET MAUI cool?
- Yes!
- Yes!
- Yes!
Wow, so it is!
`
console.log(poll);
as well as backslashes:
let poll = 'Is .NET MAUI cool? \n\
- Yes! \n\
- Yes! \n\
- Yes! \n\
Wow, so it is! \n\
'
console.log(poll);
We need to add tets for these as well.

@mattleibow , I've optimized the fix to handle both scenarios you mentioned:
Template literals with backticks for multiline strings
Backslash line continuations with embedded \n escape sequences
I've added test cases for both scenarios in WebViewHelperTests.cs to verify proper handling. The implementation correctly preserves JavaScript string semantics while ensuring proper escaping for WebView evaluation.

As recommended, I've worked these changes on top of PR #27528 and updated this PR to include those changes. This approach ensures my PR can be smoothly merged after PR #27528 is completed.

MartyIX · 2025-07-02T14:25:55Z

FYI: I opened this issue for Windows' WebView: MicrosoftEdge/WebView2Feedback#5288. The thing is that this

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

Line 136 in 5b82faa

script = "try{eval('" + script + "')}catch(e){'null'};";

should not be necessary for Windows based on WebView2 documentation:

A function that has no explicit return value returns undefined. If the script that was run throws an unhandled exception, then the result is also null. This method is applied asynchronously. If the method is run after the NavigationStarting event during a navigation, the script runs in the new document when loading it, around the time ContentLoading is run. This operation works even if IsScriptEnabled is set to false.

(Emphasis mine)

Sadly, this is not how it work in practice. Anyway, we will see what the guys will say -- either there is a bug in the documentation or in the product.

Copilot

Pull Request Overview

This pull request fixes a critical issue where WebView's EvaluateJavaScriptAsync method failed to execute JavaScript code containing newline characters. The fix consolidates JavaScript string escaping logic into a shared helper class and improves newline handling by normalizing line endings and properly escaping them for cross-platform compatibility.

Consolidates duplicate JavaScript escaping logic from WebView and HybridWebView into a shared WebViewHelper class
Enhances JavaScript string processing to handle newline characters, template literals, and backslash continuations
Adds comprehensive unit tests covering various JavaScript string escaping scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/Core/src/Handlers/WebView/WebViewHelper.cs	New helper class with improved JavaScript string escaping logic that handles newlines, backticks, and complex quote scenarios
src/Core/src/Handlers/HybridWebView/HybridWebViewHandler.cs	Removes duplicate EscapeJsString method and uses the shared WebViewHelper implementation
src/Controls/src/Core/WebView/WebView.cs	Removes duplicate EscapeJsString method and uses the shared WebViewHelper implementation
src/Controls/tests/Core.UnitTests/WebViewHelperTests.cs	Comprehensive unit tests for the new JavaScript string escaping functionality

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

Copilot · 2025-09-09T07:15:29Z

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

+#endif
+
+		// Escape sequence marker
+		js = Regex.Replace(js, @"\\n", NewlineMarker);


Consider using string.Replace() instead of regex for this literal string replacement, which would be more efficient.

Suggested change

js = Regex.Replace(js, @"\\n", NewlineMarker);

js = js.Replace("\\n", NewlineMarker);

Copilot · 2025-09-09T07:15:29Z

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

+		}
+
+		// Remove backslash-newline continuation
+		js = Regex.Replace(js, @"\\[ \t]*\n", string.Empty);


This regex operation is called on every JavaScript string. Consider caching the compiled regex or using RegexOptions.Compiled for better performance in repeated calls.

This reverts commit 9272b5f.

github-actions · 2025-12-09T12:56:28Z

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.sh | bash -s -- 29126

Or

Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.ps1) } 29126"

dotnet-policy-service bot added the community ✨ Community Contribution label Apr 22, 2025

dotnet-policy-service bot added the partner/syncfusion Issues / PR's with Syncfusion collaboration label Apr 22, 2025

praveenkumarkarunanithi changed the title ~~Fix 11905~~ Fix for WebView.EvaluateJavaScriptAsync fails to execute JavaScript containing newline characters Apr 22, 2025

MartyIX reviewed Apr 22, 2025

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praveenkumarkarunanithi marked this pull request as ready for review April 22, 2025 15:28

praveenkumarkarunanithi requested a review from a team as a code owner April 22, 2025 15:28

praveenkumarkarunanithi requested review from StephaneDelcroix and tj-devel709 April 22, 2025 15:28

jsuarezruiz added the area-controls-webview WebView label Apr 23, 2025

jsuarezruiz reviewed Apr 23, 2025

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src/Controls/src/Core/WebView/WebView.cs Outdated Show resolved Hide resolved

MartyIX reviewed Apr 23, 2025

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mattleibow requested changes Jun 24, 2025

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Copilot AI review requested due to automatic review settings September 9, 2025 07:14

Copilot AI reviewed Sep 9, 2025

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praveenkumarkarunanithi closed this Sep 9, 2025

praveenkumarkarunanithi force-pushed the fix-11905 branch from 9272b5f to 0d465cf Compare September 9, 2025 07:18

praveenkumarkarunanithi added 5 commits September 9, 2025 14:04

added UI test

238e837

Update Issue11905.cs

879bea9

updated unit test

2adcc7b

updated fix including changes on PR-27528

bfc52b2

Revert "Resolved branch conflict"

5efa6b8

This reverts commit 9272b5f.

NirmalKumarYuvaraj reopened this Sep 9, 2025

praveenkumarkarunanithi and others added 3 commits September 9, 2025 15:57

fix updated.

577c243

incorporated valid copilot suggestion

5257a56

Merge branch 'dotnet:main' into fix-11905

175686c

	js = Regex.Replace(js, @"\\n", NewlineMarker);
	js = js.Replace("\\n", NewlineMarker);

Fix for WebView.EvaluateJavaScriptAsync fails to execute JavaScript containing newline characters #29126

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praveenkumarkarunanithi commented Apr 22, 2025

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