如何测试Mocha中的“抛出错误”，只使用回调（没有承诺，async / await）在异步代码中（从db读取）？

Question

I want to write some tests for a method which reads from a JSON file (simulating a db) and returns the correct name, given that exists.

This is the code I have written for my method. It does throw an error when the id is not valid.

const getOne = (id, callback) => {
    ...
    fs.readFile('db.json', (err, data) => {
      if (err) {
        throw new Error('Error reading file');
      }
      const person = JSON.parse(data)
        .filter(el => el.id === id)
        .map(el => el.name);
      if (person.length === 0) {
        throw new Error('It does not match DB entry');
      }
      callback(person);
    });
    ...

The test I have written is:

it('Should reject an invalid id', (done) => {

    api.getOne(100, (person) => {
      try {
        personFromDB = person;
      } catch (error) {

        assert.throws(() => {  
          }, new Error('It does not match DB entry'));
          //done();
      }

But it doesn't seem to pass the test. When I have the 'done()' uncommented, it passes the test, but I don't think it is because I pass the actual test, but rather because the test gets in the catch and executes the done() callback.

Any help, guidance or recommendation is much appreciated.

Answer 1

You won't be able to catch an Error being thrown in the fs.readFile callback.

Instead, pass any errors to the callback you pass to getOne .

Then you can check if an Error got passed to your callback in your test.

Here is a working example to get you started:

const fs = require('fs');
const assert = require('assert');

const api = {
  getOne: (id, callback) => {
    // ...
    fs.readFile('db.json', (err, data) => {
      if (err) return callback(err);  // <= pass err to your callback
      const person = JSON.parse(data)
        .filter(el => el.id === id)
        .map(el => el.name);
      if (person.length === 0) return callback(new Error('It does not match DB entry'));  // <= pass the Error to your callback
      callback(null, person);  // <= call the callback with person if everything worked
    })
  }
}

it('Should reject an invalid id', done => {
  api.getOne(100, (err, person) => {
    assert.strictEqual(err.message, 'It does not match DB entry');  // Success!
    done();
  });
});