From b97fd5938125ba9db7120e2f9f99b278e8d7cab0 Mon Sep 17 00:00:00 2001
From: Stephan Hoyer <ste.hoyer@gmail.com>
Date: Sat, 29 Jan 2022 13:16:04 +0100
Subject: [PATCH] Warn about reusing mutated attrs object - fixes #2719

---
 render/render.js                |  3 +++
 render/tests/test-attributes.js | 17 +++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/render/render.js b/render/render.js
index 5d5da438..9f1dae3e 100644
--- a/render/render.js
+++ b/render/render.js
@@ -800,6 +800,9 @@ module.exports = function($window) {
 		if ("selectedIndex" in attrs) setAttr(vnode, "selectedIndex", null, attrs.selectedIndex, undefined)
 	}
 	function updateAttrs(vnode, old, attrs, ns) {
+		if (old && old === attrs) {
+			console.warn("Don't reuse attrs object, use new object for every redraw, this will throw in next major")
+		}
 		if (attrs != null) {
 			// If you assign an input type that is not supported by IE 11 with an assignment expression, an error will occur.
 			//
diff --git a/render/tests/test-attributes.js b/render/tests/test-attributes.js
index 14a3b406..7df14578 100644
--- a/render/tests/test-attributes.js
+++ b/render/tests/test-attributes.js
@@ -756,4 +756,21 @@ o.spec("attributes", function() {
 			o(succeeded).equals(true)
 		})
 	})
+	o.spec("mutate attr object", function() {
+		o("warn when reusing attrs object", function() {
+			const _consoleWarn = console.warn
+			console.warn = o.spy()
+
+			const attrs = {className: "on"}
+			render(root, {tag: "input", attrs})
+
+			attrs.className = "off"
+			render(root, {tag: "input", attrs})
+
+			o(console.warn.callCount).equals(1)
+			o(console.warn.args[0]).equals("Don't reuse attrs object, use new object for every redraw, this will throw in next major")
+
+			console.warn = _consoleWarn
+		})
+	})
 })