Hydration crashes in if branch for undefined data #15819
Description
Describe the bug
Hydration can crash at
because data is undefined.When browser extension (or browser itself in some cases*) modifies the DOM content before the hydration executes, the value of data can become undefined. The result is that the whole page disappears which is unacceptable. Adding a check for undefined here fixes the issue but I am not certain what would be the desired value in this case.
*Safari WebView for example can detect some content as being telephone number and will modify it to become a link.
Reproduction
I created simple browser extension to modify #results of my application.
// manifest.json
{ "manifest_version": 3, "name": "Localhost DOM Modifier", "version": "1.0", "description": "Modifies DOM for pages running on localhost", "permissions": ["activeTab", "scripting"], "host_permissions": ["http://localhost/*"], "content_scripts": [ { "matches": ["http://localhost/*"], "js": ["content.js"], "run_at": "document_start" } ] } // content.js
window.addEventListener("load", function () { // Get #results container and add a test message as first child const resultsContainer = document.getElementById("results"); const testMessage = document.createElement("div"); testMessage.textContent = "TEST: This is a test message"; resultsContainer.insertBefore(testMessage, resultsContainer.firstChild); }); Logs
System Info
Binaries: Node: 20.17.0 npm: 11.2.0 pnpm: 9.11.0 Browsers: Brave Browser: 135.1.77.100 Chrome: 135.0.7049.95 npmPackages: svelte: 5.28.2 => 5.28.Severity
blocking all usage of svelte