Alternating responses to the same mock server endpoint

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My question:
Hi, long story short is I’m looking for a way to alternate the return value of a mock server endpoint from one request to the other. My mock server is based on an API my app service is expected to hit; it will repeatedly hit a specific endpoint for data until the return ID eventually returns as a blank string rather than a Guid (see below), providing an exit condition for my loop.


I have created an example for the endpoint where a ‘returnUuid’ collection variable is returned, and written a pre-requisite script that will update the value of that variable based on its current value, alternating between a new random guid and an empty string.

Hitting the endpoint repeatedly in Postman shows that this does work; however hitting it from either a browser or from my app shows the value not changing from whatever it happens to currently be set to. I may be making the incorrect assumption that scripts would run in this situation. If so then is there another way to acheive what I’m looking for here?

Hi @benigoat

Assuming my understanding of your description is correct…
“The pre-request script won’t run if you hit the API from a browser or app”.
The pre-request is executed inside Postman before the API call.

Hi @w4dd325

I just need some clarification on when you say ‘inside Postman’; So if I set up a simple mock server endpoint with a PR script logging something to the console, and hit that URL through a browser, I don’t see anything in the console. Is this correct?

Correct, as far as my understanding goes, a browser/app submitting the API call will not trigger the pre-req or test scripts. These live inside Postman and would be triggered when the “Send” button is clicked.