How to test Gulp tasks

My approach is to create a test instance and use exec and yeoman-assert. Though it feels more like an integration test, I find it helpful to be sure the tasks run properly (my use case being a yeoman-generator). Some (mocha) example:

'use strict';
var path = require('path');
var helpers = require('yeoman-generator').test;
var assert = require('yeoman-generator').assert;
var exec = require('child_process').exec;
var fs = require('fs');
var injectStyles = require('../.test-instance/tasks/dev');


describe('gulp inject', function ()
{
    var instancePath = path.join(__dirname, '../.test-instance');
    var mainScss = path.join(instancePath, 'app/styles/main.scss');
    var gulpfile = path.join(instancePath, 'gulpfile.js');
    var gulp = '$(which gulp)';
    var injectStylesCmd = gulp + ' injectStyles';

    describe('scss partials in styles folder', function ()
    {
        var expectedContent = [
            [mainScss, /_variables/],
            [mainScss, /base\/_buttons\.scss/],
            [mainScss, /base\/_fonts\.scss/],
            [mainScss, /base\/_forms\.scss/],
            [mainScss, /base\/_icons\.scss/],
            [mainScss, /base\/_lists\.scss/],
            [mainScss, /base\/_page\.scss/],
            [mainScss, /base\/_tables\.scss/],
            [mainScss, /base\/_typography\.scss/],
            [mainScss, /functions\/_some-function\.scss/],
            [mainScss, /mixins\/_some-mixin\.scss/],
            [mainScss, /placeholders\/_some-placeholder\.scss/]
        ];
        var expected = [
            mainScss
        ];


        beforeEach(function (done)
        {
            this.timeout(10000);
            fs.truncateSync(mainScss);
            fs.writeFileSync(mainScss, '// inject:sass\n\n// endinject');
            exec(injectStylesCmd + ' injectStyles', {
                cwd: instancePath
            }, function (err, stdout)
            {
                done();
            });
        });

        it('creates expected files', function ()
        {
            assert.file([].concat(
                expected
            ));
            assert.fileContent([].concat(
                expectedContent
            ));
        });
    });
});

Of course you need to make sure, that you have a test-instance set up. You can create your testing files via fs.writeFileSync for example. In most cases you need to make sure that the instance has the same directory structure and that at the very least the gulpfile is present.