如何使用 Mocha/Sinon/Chai 对未定义的 localStorage 进行单元测试

Question

我有 2 个简单的方法来抽象读取和写入 localStorage：

_readLocalStorage: function(key) {
    if (window.localStorage && window.localStorage.getItem(key)) {
        return JSON.parse(window.localStorage.getItem(key));
    } else {
        throw new Error('Could not read from localStorage');
    }
},

_writeLocalStorage: function(key, data) {
    try {
        window.localStorage.setItem(key, JSON.stringify(data));
    } catch (e) {
        throw new Error('Could not write to localStorage');
    }
},

显然，存根 window.localStorage.getItem/setItem 很简单。但是 localStorage 未定义的情况呢？

我试过caching/unhinging window.localStorage（第二个断言）：

describe('#_readLocalStorage', function() {
    it('should read from localStorage', function() {
        // set up
        var stub1 = sinon.stub(window.localStorage, 'getItem')
        .returns('{"foo": "bar"}');

        // run unit
        var result = service._readLocalStorage('foo');

        // verify expectations
        expect(result)
        .to.eql({foo: 'bar'});

        // tear down
        stub1.restore();
    });

    it('should throw an error if localStorage is undefined', function() {
        // set up
        var cachedLocalStorage = window.localStorage;
        window.localStorage = undefined;

        // run unit/verify expectations
        expect(service._readLocalStorage('foo'))
        .to.throw(new Error('Could not write to localStorage'));

        // tear down
        window.localStorage = cachedLocalStorage;
    });
});

但这不起作用。 Mocha/Chai 似乎没有捕捉到抛出的错误。

我环顾四周，但找不到任何方法来处理这个问题。

Answer 1

你的 expect 应该是

expect(service._readLocalStorage.bind(service, 'foo'))
    .to.throw(new Error('Could not write to localStorage'));

在调用 expect 之前代码调用 service._readLocalStorage('foo') 的方式。所以它引发了 expect 无法处理的异常。 expect 需要能够处理异常的是一个函数，expect 本身将调用它。使用 service._readLocalStorage.bind(service, 'foo') 创建一个新函数，当不带参数调用时（如 expect 所做的那样）将等同于调用 service._readLocalStorage('foo').

您的测试还有另一个问题：您的清理代码永远不会执行。断言库通过引发 JavaScript 异常来报告问题。因此，任何跟在失败异常之后的代码都不会运行除非异常被特殊处理。你可以这样做：

it('should throw an error if localStorage is undefined', function() { // set up var cachedLocalStorage = window.localStorage; window.localStorage = undefined; // run unit/verify expectations try { expect(...)...; expect(...)...; ... } finally { // tear down window.localStorage = cachedLocalStorage; } });

对于更复杂的情况，您应该使用 before、beforeEach、after、afterEach 进行设置和拆卸。

如何使用 Mocha/Sinon/Chai 对未定义的 localStorage 进行单元测试

How do I unit test localStorage being undefined with Mocha/Sinon/Chai

unit-testing

mocha.js

local-storage

sinon

chai