Unit Test a Node.js application with Mocha, Chai, and Sinon

Question

I am new to unit testing Node.js application. My application converts CSV file to JSON after some filtering.

var fs = require('fs');
var readline = require('readline');

module.exports = ((year) => {
if (typeof year !== "number" || isNaN(year)){
    throw new Error("Not a number");
}
var rlEmitter = readline.createInterface({
  input: fs.createReadStream('./datasmall.csv'),
  output: fs.createWriteStream('./data.json')
});

rlEmitter.on('line', function(line) {
   /*Filter CSV line by line*/
});
rlEmitter.on('close', function() {
   /*Write to JSON*/
 });
});

I want to unit test the code, particularly using Sinon spy, stub, and mock. For example spy that createInterface, and the callback for the "close" event is called only once. Similarly, the callback for the "line" event is called that number of times corresponding to the number of lines in the csv. Also, how to mock the CSV if it's not present during development time?

One test I tried is, but not sure if this is the right way:

describe("Test createInterface method of readline", function(err){
    it("should be called only once", function() {
        var spyCreateInterface = sinon.spy(readline, 'createInterface');
        convert(2016);
        readline.createInterface.restore();
        sinon.assert.calledOnce(spyCreateInterface);
});

Additional suggestion on proper unit test to make this code robust will be highly appreciated.

Answer 1

As you're trying to test a module that's require d by your module, you may wish to use something like rewire to "rewire" the require call.

var rewire = require("rewire");
var sinon = require("sinon");
var myModule = rewire("path/to/module");

describe("Test createInterface method of readline", function(err){
  it("should be called only once", function() {
    var readlineStub = sinon.stub();
    myModule.__set__("readline", readlineStub);
    myModule.convert(2016);
    sinon.assert.calledOnce(spyCreateInterface);
  });
});