test: refactor event-emitter-check-listener-leaks

* add block-scoping
* use common.mustCall() on callbacks that should not execute

PR-URL: https://github.com/nodejs/node/pull/13164
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-event-emitter-check-listener-leaks.js

@@ -25,73 +25,79 @@ const common = require('../common');
 const assert = require('assert');
 const events = require('events');
 
-let e = new events.EventEmitter();
-
 // default
-for (let i = 0; i < 10; i++) {
-  e.on('default', common.noop);
-}
-assert.ok(!e._events['default'].hasOwnProperty('warned'));
-e.on('default', common.noop);
-assert.ok(e._events['default'].warned);
+{
+  const e = new events.EventEmitter();
 
-// symbol
-const symbol = Symbol('symbol');
-e.setMaxListeners(1);
-e.on(symbol, common.noop);
-assert.ok(!e._events[symbol].hasOwnProperty('warned'));
-e.on(symbol, common.noop);
-assert.ok(e._events[symbol].hasOwnProperty('warned'));
+  for (let i = 0; i < 10; i++) {
+    e.on('default', common.mustNotCall());
+  }
+  assert.ok(!e._events['default'].hasOwnProperty('warned'));
+  e.on('default', common.mustNotCall());
+  assert.ok(e._events['default'].warned);
 
-// specific
-e.setMaxListeners(5);
-for (let i = 0; i < 5; i++) {
-  e.on('specific', common.noop);
-}
-assert.ok(!e._events['specific'].hasOwnProperty('warned'));
-e.on('specific', common.noop);
-assert.ok(e._events['specific'].warned);
+  // symbol
+  const symbol = Symbol('symbol');
+  e.setMaxListeners(1);
+  e.on(symbol, common.mustNotCall());
+  assert.ok(!e._events[symbol].hasOwnProperty('warned'));
+  e.on(symbol, common.mustNotCall());
+  assert.ok(e._events[symbol].hasOwnProperty('warned'));
+
+  // specific
+  e.setMaxListeners(5);
+  for (let i = 0; i < 5; i++) {
+    e.on('specific', common.mustNotCall());
+  }
+  assert.ok(!e._events['specific'].hasOwnProperty('warned'));
+  e.on('specific', common.mustNotCall());
+  assert.ok(e._events['specific'].warned);
 
-// only one
-e.setMaxListeners(1);
-e.on('only one', common.noop);
-assert.ok(!e._events['only one'].hasOwnProperty('warned'));
-e.on('only one', common.noop);
-assert.ok(e._events['only one'].hasOwnProperty('warned'));
+  // only one
+  e.setMaxListeners(1);
+  e.on('only one', common.mustNotCall());
+  assert.ok(!e._events['only one'].hasOwnProperty('warned'));
+  e.on('only one', common.mustNotCall());
+  assert.ok(e._events['only one'].hasOwnProperty('warned'));
 
-// unlimited
-e.setMaxListeners(0);
-for (let i = 0; i < 1000; i++) {
-  e.on('unlimited', common.noop);
+  // unlimited
+  e.setMaxListeners(0);
+  for (let i = 0; i < 1000; i++) {
+    e.on('unlimited', common.mustNotCall());
+  }
+  assert.ok(!e._events['unlimited'].hasOwnProperty('warned'));
 }
-assert.ok(!e._events['unlimited'].hasOwnProperty('warned'));
 
 // process-wide
-events.EventEmitter.defaultMaxListeners = 42;
-e = new events.EventEmitter();
+{
+  events.EventEmitter.defaultMaxListeners = 42;
+  const e = new events.EventEmitter();
 
-for (let i = 0; i < 42; ++i) {
-  e.on('fortytwo', common.noop);
-}
-assert.ok(!e._events['fortytwo'].hasOwnProperty('warned'));
-e.on('fortytwo', common.noop);
-assert.ok(e._events['fortytwo'].hasOwnProperty('warned'));
-delete e._events['fortytwo'].warned;
+  for (let i = 0; i < 42; ++i) {
+    e.on('fortytwo', common.mustNotCall());
+  }
+  assert.ok(!e._events['fortytwo'].hasOwnProperty('warned'));
+  e.on('fortytwo', common.mustNotCall());
+  assert.ok(e._events['fortytwo'].hasOwnProperty('warned'));
+  delete e._events['fortytwo'].warned;
 
-events.EventEmitter.defaultMaxListeners = 44;
-e.on('fortytwo', common.noop);
-assert.ok(!e._events['fortytwo'].hasOwnProperty('warned'));
-e.on('fortytwo', common.noop);
-assert.ok(e._events['fortytwo'].hasOwnProperty('warned'));
+  events.EventEmitter.defaultMaxListeners = 44;
+  e.on('fortytwo', common.mustNotCall());
+  assert.ok(!e._events['fortytwo'].hasOwnProperty('warned'));
+  e.on('fortytwo', common.mustNotCall());
+  assert.ok(e._events['fortytwo'].hasOwnProperty('warned'));
+}
 
 // but _maxListeners still has precedence over defaultMaxListeners
-events.EventEmitter.defaultMaxListeners = 42;
-e = new events.EventEmitter();
-e.setMaxListeners(1);
-e.on('uno', common.noop);
-assert.ok(!e._events['uno'].hasOwnProperty('warned'));
-e.on('uno', common.noop);
-assert.ok(e._events['uno'].hasOwnProperty('warned'));
+{
+  events.EventEmitter.defaultMaxListeners = 42;
+  const e = new events.EventEmitter();
+  e.setMaxListeners(1);
+  e.on('uno', common.mustNotCall());
+  assert.ok(!e._events['uno'].hasOwnProperty('warned'));
+  e.on('uno', common.mustNotCall());
+  assert.ok(e._events['uno'].hasOwnProperty('warned'));
 
-// chainable
-assert.strictEqual(e, e.setMaxListeners(1));
+  // chainable
+  assert.strictEqual(e, e.setMaxListeners(1));
+}