How to test for an event to have been fired and its value?

Question

I couldn't find any working examples to test for whether an event was emitted and whether the emitted value is as expected.

Here is the class that emits messages and its parent:

const EventEmitter = require('events').EventEmitter;

class FileHandler extends EventEmitter {
            constructor() {
                super();
            }

            canHandle(filePath) {
                emit('not super type');
            }

            parseFile(filePath) {
                emit('supper parsing failed');
            }

            whoAmI() {
                return this.emit('who',"FileHandler");
            }
        }

module.exports = FileHandler;

//diff file

const FileHandler = require('./FileHandler');

class FileHandlerEstedamah extends FileHandler {
            constructor() {
                super();
            }

            canHandle(filePath) {
                this.emit('FH_check','fail, not my type');
            }

            parseFile(filePath) {
                this.emit('FH_parse','success');
            }
        }

module.exports = FileHandlerEstedamah;

Here is my current test code:

var sinon = require('sinon');
var chai = require('chai');

const FileHandlerEstedamah = require("../FileHandlerEstedamah");    

describe('FH_parse', function() {    
    it('should fire an FH_parse event', function(){
        const fhe = new FileHandlerEstedamah(); 
        var fhParseSpy = sinon.spy();
        fhe.on('FH_parse',fhParseSpy);       
        fhe.parseFile("path");

        //I tried a large number of variants of expect, assert, etc to no avail.
    });
});

I expected this to be straightforward but somehow I am missing something.

Thank you, Jens

Answer 1

You almost nearly there. To check the value, we must call done() after that in mocha to tell that our test is finish.

The code

const chai = require('chai');
const assert = chai.assert;
const sinon = require('sinon');
const FileHandlerEstedamah = require("../FileHandlerEstedamah");

describe('FH_parse', function() {    
  it('should fire an FH_parse event', function(done){ // specify done here
      const fhe = new FileHandlerEstedamah(); 
      fhe.on('FH_parse', (data) => {
        assert.equal(data, 'success'); // check the value expectation here
        done(); // hey mocha, I'm finished with this test
      });       
      fhe.parseFile("path");      
  });
});

Hope it helps