js: Use jQuery as a module.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/zjsunit/index.js

@@ -12,8 +12,7 @@ const stub_i18n = require("./i18n");
 const namespace = require("./namespace");
 const test = require("./test");
 const {make_zblueslip} = require("./zblueslip");
-
-global.$ = require("./zjquery");
+const zjquery = require("./zjquery");
 
 require("@babel/register")({
     extensions: [".es6", ".es", ".jsx", ".js", ".mjs", ".ts"],
@@ -71,7 +70,7 @@ function short_tb(tb) {
 }
 
 function run_one_module(file) {
-    global.$.clear_all_elements();
+    zjquery.clear_all_elements();
     console.info("running test " + path.basename(file, ".js"));
     test.set_current_file_name(file);
     require(file);

frontend_tests/zjsunit/namespace.js

@@ -12,12 +12,20 @@ let actual_load;
 const module_mocks = new Map();
 const used_module_mocks = new Set();
 
+const jquery_path = require.resolve("jquery");
+const real_jquery_path = require.resolve("../zjsunit/real_jquery.js");
+
 function load(request, parent, isMain) {
     const filename = Module._resolveFilename(request, parent, isMain);
     if (module_mocks.has(filename)) {
         used_module_mocks.add(filename);
         return module_mocks.get(filename);
     }
+    if (filename === jquery_path && parent.filename !== real_jquery_path) {
+        // jQuery exposes an incompatible API to Node vs. browser, so
+        // this wouldn't work.
+        throw new Error("This test will need jquery mocked using zjquery or real_jquery");
+    }
 
     return actual_load(request, parent, isMain);
 }

frontend_tests/zjsunit/real_jquery.js

@@ -0,0 +1,8 @@
+"use strict";
+
+const jquery = require("jquery");
+
+// so the tests can mock jQuery
+delete require.cache[require.resolve("jquery")];
+
+module.exports = jquery;