Mocking & E2E Tests — Series 3

Preview — 3 of 10 questions

When is the factory form preferable to useValue?

javascript

.overrideProvider(MailService)
.useFactory({
  factory: (config: ConfigService) => new RecordingMailer(config.get('MAIL_FROM')),
  inject: [ConfigService],
})

AAlways, since useValue cannot provide an object with methods

BWhen the double itself needs something from the container — configuration, a shared recorder, another test helper — because useFactory is resolved through DI with its own inject list, while useValue can only supply an object you built by hand

CWhen the override must apply to several tests at once

DWhen the original provider is request-scoped

What does the second approach cover that the first does not?

javascript

// (1) call intercept() directly with a fabricated context and CallHandler
// (2) register it on a route and assert on the HTTP response

ANothing; the direct call is strictly more precise

BOnly the response status, which the direct call cannot produce

CThe interceptor's internal logic, which is inaccessible otherwise

DThat it is actually bound where you think, that it composes correctly with the other enhancers on that route, and that its transformation survives serialisation — the parts most likely to be wrong, since the interceptor's own logic is usually the simple half

When is this appropriate?

javascript

expect(response.body).toMatchSnapshot();

AWhen the shape is large and genuinely stable, and you want any change to it to be visible in review — the snapshot then acts as a reviewed contract; it works badly when the payload contains ids, timestamps or ordering that vary, since every run produces a diff and people learn to update snapshots reflexively

BAlways, since a snapshot covers more of the response than hand-written assertions

COnly for responses under 1 kB, which is Jest's limit

DNever — snapshots are unsuitable for API responses

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