How to stub a nodejs "required" constructor using sinon? - node.js

Question

I'm writing unit tests for a method that uses the email-templates module like this:
var EmailTemplate = require('email-templates').EmailTemplate;
module.exports = {
sendTemplateEmail: function (emailName, data, subject, to, from) {
var template = new EmailTemplate(__dirname + "/../emails/" + emailName);
data.from = FROM;
data.host = config.host;
return template.render(data)
.then(function (result) {
return mailer.sendEmail(subject, to, from, result.html, result.text);
})
.then(function () {
log.info(util.format("Sent %s email to %s. data=%s", emailName, to, JSON.stringify(data)));
return Promise.resolve();
})
.catch(function (err) {
return Promise.reject(new InternalError(err, "Error sending %s email to %s. data=%s", emailName, to, JSON.stringify(data)));
});
}
};
The unit test looks like this:
var assert = require("assert"),
sinon = require("sinon"),
Promise = require("bluebird"),
proxyquire = require("proxyquire");
describe('mailer#sendTemplateEmail', function () {
var templates,
template;
beforeEach(function() {
templates = {
EmailTemplate: function(path) {}
};
template = {
render: function(data) {}
};
sinon.stub(templates, "EmailTemplate").returns(template);
});
it("should reject immediately if template.render fails", function () {
const TO = {email: "user1#example.com", first: "User"};
const FROM = {email: "user2#example.com", first: "User"};
const EMAIL_NAME = "results";
const SUBJECT = "Results are in!";
const DATA = {
week: 10,
season: "2015"
};
var err = new Error("error");
var mailer = proxyquire("../src/mailer", {
"email-templates": templates
});
sinon.stub(template, "render").returns(Promise.reject(err));
return mailer.sendTemplateEmail(EMAIL_NAME, DATA, SUBJECT, TO, FROM)
.then(function () {
assert.fail("Expected a rejected promise.");
})
.catch(function (err) {
assert(err.message === "error");
assert(mailer.sendEmail.notCalled);
});
});
};
The problem I'm encountering is on the first line of the sendTemplateEmail function which instantiates a new EmailTemplate object. The EmailTemplate constructor being called points to the non-stub EmailTemplate function defined in the beforeEach, rather than the sinon stub created on the last line of the beforeEach. If I evaluate the require('email-templates').EmailTemplate statement, however, it correctly points to the sinon stub. I'd prefer not to have to change my code to call the require statement inline like:
var template = new require('email-templates').EmailTemplate(__dirname + "/../emails/" + emailName);
Is there any way to accomplish the stub the way I'm intending?

Answer 1

You can inject your dependency when you construct your mailer - exp:
function mailer(options) {
options = options || {};
this.email_template = options.email_template;
}
Then in the sendTemplateEmail function - use the email_template member.
Also - not sure about your mailer code - but if you need your mailer to act as a singleton in your code (and it isn't already) - you can add this to your mailer:
module.exports = {
getInstance: function(emailTemplate) {
if(this.instance === null){
this.instance = new mailer(emailTemplate);
}
return this.instance;
}
}
Then when you require your mailer you can use the syntax:
var template = new require('email-templates').EmailTemplate(__dirname + "/../emails/" + emailName);
var mail = mailer.getInstance(template);
This way your application (unit test framework or your actual/real-world application) will determine the type of mailer that will be used for the lifetime of the process.