Sinon stub private variable in typescript? - unit-testing

Question

I would like to stub a private variable inside a class
class IPC {
private publisher: redis.RedisClient;
constructor() {
this.publisher = redis.createClient();
}
publish(text: string) {
const msg = {
text: text
};
this.publisher.publish('hello', JSON.stringify(msg));
}
}
How can I stub the private variable publisher , inside this class?
so I could test the code as shown below
it('should return text object', () => {
const ipc = sinon.createStubInstance(IPC);
ipc.publish('world!');
// this will throw error, because ipc.publisher is undefined
assert.deepStrictEqual({
text: 'world!'
}, ipc.publisher.getCall(0).args[0])
})

Answer 1

You can use type assertion in order to get access to the private variable. Like:
(ipc as any).publisher

Answer 2

There is no way to stub a private variable, and this is not the right way to do it, you could see the discussion below with Christian Johansen
https://groups.google.com/forum/#!topic/sinonjs/ixtXspcamg8
The best approach, is to inject any dependency into the constructor, once we refactor the code, we could easily stub the dependency with our required behaviour
class IPC {
constructor(private publisher: redis.RedisClient) {
}
publish(text: string) {
const msg = {
text: text
};
this.publisher.publish('hello', JSON.stringify(msg));
}
}
it('should return text object', () => {
sinon.stub(redis, 'createClient')
.returns({
publish: sinon.spy()
});
const publisherStub = redis.createClient();
const ipc = new IPC(publisherStub)
ipc.publish('world!');
// this is working fine now
assert.deepStrictEqual({
text: 'world!'
}, publisherStub.publish.getCall(0).args[0])
sinon.restore(redis.createClient)
})