NodeJS testing writeFile with mock-fs

Question

For example, I have the following file:

spec/config/file-mock.js

var mock = require('mock-fs');
mock({
  'files': {
    'expense.csv': 'a;b;c;d\n1;2;3;4\n5;6;7;8'
  }
});

app/reader/reader.js

var fs = require('fs');
var reader = {
    read: function(path) {
        return fs.readFileSync(path, 'utf8');
    },
    write: function(path, object) {
        fs.writeFileSync(path, object);
    }
};
module.exports = reader;

app/reader/reader.spec.js

describe('reader.js test', function(){
    var reader = require('./reader.js');
    var mock = require('mock-fs');
    it('should return a simple string', function(){
        expect(reader.read('files/expense.csv')).toEqual('a;b;c;d\n1;2;3;4\n5;6;7;8');
    });
    it('should write a json object', function(){
        // WHAT TO DO?!
    });
});

The reader.read function is working fine with mock-fs.

But I'm trying to test the reader.write function and I didn't found anything in the documentation about the "write" functions. Anyone already solved this?

Answer 1

As @Strech posted in comments you might try an expect on reader.read after calling reader.write to check that the content has been written to the fake file.

IMHO there is a better way to test this functionality out a bit more stick to unit test principles. We know that fs module works because is part of node core modules and it's tested there, so we can mock fs methods and instead check that those methods are being called with the expected parameters, for instance using Sinon:

const Reader = require('../reader.js');
const sinon = require('sinon');
const fs = require('fs');
const expect = require('expect.js');

describe('reader.js test', function () {

  let readFileSync;
  let writeFileSync;

  beforeEach(() => {
    readFileSync = sinon.stub(fs, 'readFileSync').returns({});
    writeFileSync = sinon.stub(fs, 'writeFileSync').returns({});
  });
  afterEach(() => {
    readFileSync.restore();
    writeFileSync.restore();
  });

  it('should return a simple string', () => {

    Reader.read('files/expense.csv');
    expect(readFileSync.calledOnceWith('files/expense.csv', 'utf8')).to.be(true);
  });

  it('should write a json object', () => {

    Reader.write('files/expense.csv', 'test');
    expect(writeFileSync.calledOnceWith('files/expense.csv', 'test')).to.be(true);
  });
});