我尝试存根任何全局函数：（function randomNumber(min, max){/*...*/}）window.randomNumber === randomNumber但是当我在测试用例中创建存根sinon.stub(window, 'randomNumber')并模拟结果randomNumber.returns(1);)时它不起作用并调用原始的'randomNumber'（window.randomNumber !== randomNumber），为什么？
[编辑]
我的部分代码：https ://plnkr.co/edit/GAaxA3iN8QehDN7HbOc8?p=preview （有趣的是它在浏览器中，在控制台中运行undefined is not a function (evaluating 'randomNumber.onCall(0)')）和package.json：
"chai": "^3.5.0",
"karma": "^1.7.0",
"karma-mocha": "^1.3.0",
"karma-phantomjs-launcher": "^1.0.4",
"karma-sinon-chai": "^1.3.1",
"mocha": "^3.4.1",
"phantomjs-prebuilt": "^2.1.14",
"sinon": "^2.2.0",
"sinon-chai": "^2.10.0",
"sinon-stub-promise": "^4.0.0"