How do you replace Angular dependencies in a TestBed test?

Overrides dependencies at the provider boundary while preserving the same injection scope and contract used in production.

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TL;DR

I configure TestBed with the component and provide a fake or spy for the service or injection token it consumes. The fake implements the smallest public contract needed by the test and returns controlled values or failures. I retrieve dependencies from TestBed or the fixture injector rather than constructing a second instance that the component never uses.

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TestBed, dependency injection, provider overrides, injection tokens, environment providers, and test isolation.

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Core answer
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Overview
A replacement is useful only if the component receives it through the same dependency boundary it uses in production.
I configure TestBed with the component and provide a fake or spy for the service or injection token it consumes. The fake implements the smallest public contract needed by the test and returns controlled values or failures. I retrieve dependencies from TestBed or the fixture injector rather than constructing a second instance that the component never uses.
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Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
Provider location matters because an application provider, route provider, and component provider can create different instances. If the component declares its own provider, a root-level fake may not replace it, so I override the component metadata or obtain the scoped instance deliberately. For functional code that calls inject, I run it in an injection context instead of rewriting production code around the test. Each test resets mutable fake state so order does not affect results.

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Details

Constructing a spy that is not the instance Angular injected.
Ignoring a component-level provider that shadows the test provider.
Sharing mutable fake state across tests.

How do you replace Angular dependencies in a TestBed test?

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