I'm an BE engineer with experience in BE unit testing too, but recently started working on FE end-to-end tests, using Playwright and Cucumber. Should the expected result of a scenario be a hard-coded example or should it be calculated as part of the test?
For simplification, let's say I'm trying to write a test for a discount on an e-commerce site, and I'm writing a test to verify the discount amount is correct. It takes the user's basket total as an input and performs a client-side calculation to apply a fixed 12% discount, and spits the result back to the user. Should I be calculating that 12% within the step definitions of the test? Or should I hard-code the expected result? Feature file would look something like this if I was hard-coding the expected result:
Scenario: Verify discount amount is correct
Given the shopping basket total is 60.00
When I click "Calculate Discount"
Then the reduction should be 7.20
Or should the last line be changed to calculate it like Then the reduction should be as expected and the step definition file doing something like
@Then("the reduction should be as expected")
public void theReductionShouldBe() {
double actual = getReductionDisplayedToUser();
double expected = basketTotal * 0.12;
assertEquals(expected, actual);
}
In reality the calculation for the tool I'm testing is more complicated than just working out 12% (there's several inputs instead of just one), so my issue with the latter is that I'm duplicating all the logic in my e2e testing repo. But there's an argument that if the logic should change, I don't need to touch my feature file, and can just update the logic in the calculation. I personally prefer the former as it's similar to the testing style of my BE unit tests, but not sure if that's the best practice with E2E tests.
Another colleague has suggested I implement randomness to the inputs (and that would only work with the calculated result), but it seems strange to me to theoretically have a test that sometimes fails and sometimes passes if there is a bug in the logic, making maintenance a bit more difficult such as checking the bug as been fixed after a failed test.
Scenario: Verify discount amount is correct
Given the shopping basket total is a random amount between 0.00 and 1000.00
When I click "Calculate Discount"
Then the reduction should be as expected
As I said I'm a bit new to E2E testing, and I'm not sure if there's any standards or best practices with this. Any thoughts?
If you follow cucumber official scenario example document of 'Writing better Gherkin', they have written expected values in Then condition
Official Cucumber document for Writing better Gherkin
You can also follow Scenario Outline where you can test multiple conditions
Scenario Outline: Verify discount amount is correct
Given the shopping basket total is <total>
When I click "Calculate Discount"
Then the reduction should be <expected>
Examples:
| total | expected |
| 60.00 | 7.20 |
| -1 | 0 |
And then you can play within different examples