How do you test an Angular service that uses HttpClient?

Uses HttpTestingController to verify request semantics and return controlled responses without calling a real server.

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TL;DR

I configure the HTTP testing backend, call the service method, and use HttpTestingController to expect the outgoing request. I verify the method, URL, query, headers, or body that belong to the service contract, then flush a controlled response and assert the public result. After the test I verify that no unexpected requests remain.

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HttpClient testing, HttpTestingController, request matching, response flushing, errors, and outstanding-request verification.

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Core answer
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Overview
An HttpClient service test should verify the browser-to-API contract without becoming a backend integration test.
I configure the HTTP testing backend, call the service method, and use HttpTestingController to expect the outgoing request. I verify the method, URL, query, headers, or body that belong to the service contract, then flush a controlled response and assert the public result. After the test I verify that no unexpected requests remain.
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Stronger answer
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I also cover relevant transport failures and backend error responses, including how the service maps them for its consumers. Interceptors can be included when their behavior is part of the test, but most feature-service tests should not duplicate every global middleware assertion. For concurrent requests I use match and complete them in a deliberate order. I avoid asserting incidental header ordering or serialized details that are not contractually meaningful.

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Details

Calling a real API from a unit test.
Flushing the response before asserting the outgoing request.
Forgetting to verify outstanding requests.

How do you test an Angular service that uses HttpClient?

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