zjquery: Prevent direct patches to $.

We prohibit code from making direct patches to
$ unless you use override().

frontend_tests/zjsunit/namespace.js

@@ -122,14 +122,31 @@ exports.with_overrides = function (test_function) {
 
         unused_funcs.get(obj).set(func_name, true);
 
-        const old_f = obj[func_name];
-        obj[func_name] = function (...args) {
+        let old_f = obj[func_name];
+        if (old_f === undefined) {
+            // Create a dummy function so that we can
+            // attach _patched_with_override to it.
+            old_f = () => {
+                throw new Error(`There is no ${func_name}() field for this object.`);
+            };
+        }
+
+        const new_f = function (...args) {
             unused_funcs.get(obj).delete(func_name);
             return f.apply(this, args);
         };
 
+        // Let zjquery know this function was patched with override,
+        // so it doesn't complain about us modifying it.  (Other
+        // code can also use this, as needed.)
+        new_f._patched_with_override = true;
+
+        obj[func_name] = new_f;
+
         restore_callbacks.push(() => {
+            old_f._patched_with_override = true;
             obj[func_name] = old_f;
+            delete old_f._patched_with_override;
         });
     };
 

frontend_tests/zjsunit/zjquery.js

@@ -558,4 +558,32 @@ function make_zjquery() {
     return zjquery;
 }
 
-module.exports = make_zjquery();
+const $ = new Proxy(make_zjquery(), {
+    set(obj, prop, value) {
+        if (value._patched_with_override) {
+            obj[prop] = value;
+            return true;
+        }
+
+        throw new Error(`
+            Please don't modify $.${prop} if you are using zjquery.
+
+            You can do this instead:
+
+                override($, "${prop}", () => {...});
+
+            Or you can do this if you don't actually
+            need zjquery and just want to simulate one function.
+
+                set_global("$", {
+                    ${prop}(...) {...},
+                });
+
+            It's also possible that you are testing code with
+            node tests when it would be a better strategy to
+            use puppeteer tests.
+        `);
+    },
+});
+
+module.exports = $;