How to test setInterval on a method? - node.js

Question

I have a class
class Dummy {
constructor() {
this.prop1 = null;
this.prop2 = null;
this.prop3 = setInterval(() => {
this.method1()
}, 1000);
}
method1() {
// Method logic
}
}
var dummyObject = new Dummy();
module.exports = dummyObject;
I'd like to write tests to verify that method1 is being invoked after every 1s.
Following is the test code
const dummyObject = require('./dummy.js');
describe("Test setInterval", function () {
it("Test setInterval", function () {
const clock = sinon.useFakeTimers();
const spy = sinon.spy(dummyObject, 'method1');
clock.tick(1001);
expect(spy.calledOnce).to.be.true;
clock.restore();
})
})
When I run the tests however, I get an error 'Expected false to equal to true' and on further digging I realized I am not able to spy on the method which is being called via setInterval.
Please share any thoughts on what I can do to test this scenario?

Answer 1

This is not going to work the way you want it to, because the method (method1) is already called when you require the module and hence there is no chance to spy it afterwards in your test.
I recommend to refactor your Module to export the class, not the instance like:
module.exports = class Dummy {
constructor() {
this.prop1 = null;
this.prop2 = null;
this.prop3 = setInterval(() => {
this.method1()
}, 1000);
}
method1() {
// Method logic
}
}
Then in you test, require the class and spy on the method before you instantiate it:
const sinon = require('sinon');
const Dummy = require('./dummy.js');
describe("Test setInterval", function () {
it("Test setInterval", function () {
const clock = sinon.useFakeTimers();
// Spy on the method using the class' prototype
const spy = sinon.spy(Dummy.prototype, 'method1');
// Initialize the class and make sure its `constructor` is called after you spied on the method
new Dummy();
clock.tick(1001);
expect(spy.calledOnce).to.be.true;
clock.restore();
})
})