为什么我的jest.mock中的Promise reject()转到then()而不是catch()？

Question

我有两个文件,getItemInfo.js用于进行API调用,getItemInfo.test.js哪个是相应的Jest测试文件.

在测试文件中,我正在模拟由节点模块触发的http调用request-promise.

问题出在第二个代码块上,被*********s 包围.基本上为什么在第二次单元测试中reject()错误仍然存​​在then()？

// getItemInfo.js

const rp = require('request-promise');

const getItemInfo = (id) => {
    const root = 'https://jsonplaceholder.typicode.com/posts/';
    const requestOptions = {
        uri: `${root}/${id}`,
        method: 'GET',
        json: true
    }

    return rp(requestOptions)
    .then((result) => {
        return result;
    })
    .catch((err) => {
        return err;
    });
};

module.exports = {
    getItemInfo: getItemInfo
};

这是我的Jest单元测试文件.

// getItemInfo.test.js
const ItemService = require('./getItemInfo');

jest.mock('request-promise', () => (options) => {
    const id = Number.parseInt(options.uri.substring(options.uri.lastIndexOf('/') + 1));

    return new Promise((resolve, reject) => {
        if (id === 12) {
            return resolve({
                id: id,
                userId: 1,
                title: '',
                body: ''
            });
        } else {
            return reject('something went wrong'); // <-- HERE IS THE REJECT
        }
    })
});

describe('getItemInfo', () => {
    it('can pass', done => {
        const TEST_ID = 12
        ItemService.getItemInfo(TEST_ID).then((result) => {
            console.log('result:',result);
            expect(result.id).toBe(TEST_ID);
            expect(result.userId).toBeDefined();
            expect(result.title).toBeDefined();
            expect(result.body).toBeDefined();
            done();
        });
    });

    it('can fail', (done) => {
        const TEST_ID = 13;
        ItemService.getItemInfo(TEST_ID)
        .catch((err) => {
            // *************
            // This "catch" block never runs
            // even if the jest.mock above Promise.rejects
            // Why is that???
            // *************
            console.log('catch():', err);
            done();
        })
        .then((result) => {
            // this block runs instead.
            // and it returns "then: something went wrong"
            console.log('then():', result);
            done();
        });
    });
});

这是单元测试的输出.命令很简单jest.最后一行应该从catch()语句中运行,而不是then():

PASS  ./getItemInfo.test.js
 getItemInfo
   ? can pass (9ms)
   ? can fail (1ms)

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        0.703s, estimated 1s
Ran all test suites.
----------------|----------|----------|----------|----------|----------------|
File            |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------------|----------|----------|----------|----------|----------------|
All files       |      100 |      100 |      100 |      100 |                |
getItemInfo.js |      100 |      100 |      100 |      100 |                |
----------------|----------|----------|----------|----------|----------------|
 console.log getItemInfo.test.js:25
   result: { id: 12, userId: 1, title: '', body: '' }

 console.log getItemInfo.test.js:48
   then(): something went wrong

我究竟做错了什么？

Answer 1

为什么我的jest.mock中的Promise reject()转到then()而不是catch()？

您的.catch()处理程序将被拒绝的promise转换为已解析的promise,因此只.then()调用外部处理程序.

当你这样使用时.catch():

.catch((err) => {
    return err;
});

并且不要重新抛出错误或返回被拒绝的承诺,然后拒绝被视为"已处理"并且返回的承诺将被解决,而不是被拒绝.这就像使用try/catch一样.在catch处理程序中,除非再次抛出异常,否则将处理异常.

你可以在这个简单的片段中看到:

new Promise((resolve, reject) => {
    reject(new Error("reject 1"));
}).catch(err => {
    // returning a normal value here (anything other than a rejected promise)
    // causes the promise chain to flip to resolved
    return err;
}).then(val => {
    console.log("Promise is now resolved, not rejected");
}).catch(err => {
    console.log("Don't get here");
});

其中任何一个都没有理由:

.then((result) => {
    return result;
})
.catch((err) => {
    return err;
});

你可以删除它们.该.then()处理器就是多余的代码和.catch()处理吃排斥和把它变成一个解决的承诺.

如果你想保留.catch()处理程序,但允许拒绝向上传播,那么你需要重新抛出.

.catch((err) => {
    console.log(err);
    throw err;       // keep promise rejected so reject will propagate upwards
});