Nodejs test image resizing with mocha - node.js

Question

I've used graphicmagick to resize a image in my nodejs app.
The problem is when writing the unit tests, I cant seem to find any direction or examples on this. Does it make sense that I test the image resizing, seeing that I'm using a third-party module? If yes, how could I write a test for my code?
// dependencies
var async = require('async');
var AWS = require('aws-sdk');
var gm = require('gm').subClass({ imageMagick: true });
var util = require('util');
// get reference to S3 client
var s3 = new AWS.S3();
var _800px = {
width: 800,
destinationPath: "large"
};
var _500px = {
width: 500,
destinationPath: "medium"
};
var _200px = {
width: 200,
destinationPath: "small"
};
var _45px = {
width: 45,
destinationPath: "thumbnail"
};
var _sizesArray = [_800px, _500px, _200px, _45px];
var len = _sizesArray.length;
// handler for dev environment
exports.GruntHandler = function (filepath) {
console.log("Path to file is: " + filepath);
// get the file name
var srcFile = filepath.split("/").pop();
var dstnFile = "dst";
// Infer the image type.
var typeMatch = srcFile.match(/\.([^.]*)$/);
if (!typeMatch) {
console.error('unable to infer image type for key ' + srcFile);
return;
}
var imageType = typeMatch[1];
if (imageType != "jpg" && imageType != "png") {
console.log('skipping non-image ' + srcFile);
return;
}
for (var i = 0; i<len; i++) {
// Transform the image buffer in memory.
gm(filepath)
.resize(_sizesArray[i].width)
.write(dstnFile + "/" + _sizesArray[i].destinationPath + "/" + srcFile, function (err) {
if (err) {
console.log(err);
}
});
}
console.log(" grunt handler called!");
};

Answer 1

The convention when writing unit tests, is to test only the isolated unit.
All external dependencies should be stubbed/mocked, so you can check only the logic in your unit (in this case your unit is your module).
As to what to test, your unit's only public method is "GruntHandler", so this is the only method you should test, as it is the service that this unit provides to other units.
In order to replace all the module's dependencies, I like to use the proxyquire package. It replaces "require" calls with stubs you can control.
I've written an example test using the testing stack that I use personally: proxyquire, mocha, sinon for spies, chai for assertions.
Setup - put this in a "test-helper" module to require in all test files:
var chai = require('chai');
var sinonChai = require("sinon-chai");
var expect = chai.expect;
var sinon = require('sinon');
chai.use(sinonChai);
var proxyquire = require('proxyquire');
And in your test file:
require('./test-helper');
describe('Image Resizing module', function () {
var gmSubclassStub = sinon.stub();
var testedModule = proxyquire('path-to-tested-file', {
'gm': {subClass: sinon.stub().returns(gmSubclassStub)}
});
describe.only('GruntHandler', function () {
it("should call gm write with correct files", function () {
// Arrange
var filepath = 'pepo.jpg';
// Spies are the methods you expect were actually called
var write800Spy = sinon.spy();
var write500Spy = sinon.spy();
var write200Spy = sinon.spy();
var write45Spy = sinon.spy();
// This is a stub that will return the correct spy for each iteration of the for loop
var resizeStub = sinon.stub();
resizeStub.withArgs(800).returns({write:write800Spy});
resizeStub.withArgs(500).returns({write:write500Spy});
resizeStub.withArgs(200).returns({write:write200Spy});
resizeStub.withArgs(45).returns({write:write45Spy});
// Stub is used when you just want to simulate a returned value
gmSubclassStub.withArgs(filepath).returns({resize:resizeStub});
// Act - this calls the tested method
testedModule.GruntHandler(filepath);
// Assert
expect(write800Spy).calledWith("dst/large/pepo.jpg");
expect(write500Spy).calledWith("dst/medium/pepo.jpg");
expect(write200Spy).calledWith("dst/small/pepo.jpg");
expect(write45Spy).calledWith("dst/thumbnail/pepo.jpg");
});
});
});
More about sinon spies, stubs, and mocks: http://sinonjs.org/
Proxyquire: https://github.com/thlorenz/proxyquire
And a great tutorial about all this together: http://kroltech.com/2014/02/node-js-testing-with-mocha-chai-sinon-proxyquire/#.VPTS9fmUdV0