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DummyJSON
DummyJSON.com provides different types of REST Endpoints filled with JSON data which you can use in developing the frontend with your favorite framework and library without worrying about writing a backend.
DummyJSON is a set of endpoints you can use to try out different requests without using a commercial API which may or may not ban you or limit your access..
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
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I would recommend using an http client like Insomnia or Postman, or cURL from the cli to get a better understanding of the requests themselves without needing to code the requests by hand in a bloated html form.
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https://httpbin.org/ is an excellent tool for learning (and testing) the basics. Take a few minutes to explore the endpoints they provide. You can also use different response codes to test your apps error handling, etc