WIP

packages/svelte/src/internal/client/dev/log.js

@@ -0,0 +1,43 @@
+import { STATE_SYMBOL } from '../constants.js';
+
+export function monkey_patch_console() {
+	for (const method of Object.keys(console)) {
+		// @ts-expect-error
+		const original = console[method];
+
+		// @ts-expect-error
+		console[method] = (...args) => {
+			for (const arg of args) {
+				if (contains_state_proxy(arg)) {
+					// TODO make this a proper warning
+					console.warn('contains state proxy!!!!');
+					break;
+				}
+			}
+
+			return original.apply(console, args);
+		};
+	}
+}
+
+/**
+ * @param {any} value
+ * @param {Set<any>} seen
+ * @returns {boolean}
+ */
+function contains_state_proxy(value, seen = new Set()) {
+	if (typeof value !== 'object' || value === null) return false;
+
+	if (seen.has(value)) return false;
+	seen.add(value);
+
+	if (STATE_SYMBOL in value) {
+		return true;
+	}
+
+	for (const key in value) {
+		if (contains_state_proxy(value[key], seen)) {
+			return true;
+		}
+	}
+}

packages/svelte/src/internal/client/index.js

@@ -1,3 +1,6 @@
+import { DEV } from 'esm-env';
+import { monkey_patch_console } from './dev/log.js';
+
 export { FILENAME, HMR } from '../../constants.js';
 export { add_locations } from './dev/elements.js';
 export { hmr } from './dev/hmr.js';
@@ -164,3 +167,7 @@ export {
 	validate_void_dynamic_element
 } from '../shared/validate.js';
 export { strict_equals, equals } from './dev/equality.js';
+
+if (DEV) {
+	monkey_patch_console();
+}