What End-to-End Tests Are For

The top of the pyramid

An end-to-end (E2E) test opens a real browser, loads the running application and acts like a user: it clicks, types and checks what appears on screen, usually with a real or realistic backend behind it. The classic test pyramid puts many fast unit tests at the bottom, fewer integration tests in the middle and a small number of E2E tests on top; the testing trophy variant puts more weight on integration tests. Either way, E2E tests are the slowest and most expensive to maintain, so they should cover the critical user journeys (sign-up, login, checkout, the core workflow of the product) rather than every edge case. Validation rules and calculations are better tested lower down.

Test the app the way users use it

End-to-end tests drive a real browser against a running application to prove that key user journeys work.

Deciding what deserves an E2E test

A rough triage list.

# good E2E candidates (high value, crosses many layers)
#   - a new user can sign up and reach the dashboard
#   - a signed-in user can add an item to the cart and pay
#   - an admin can invite a teammate and the invite email is queued
#
# better tested lower in the stack
#   - every validation message on a form     -> component / unit tests
#   - price calculation edge cases           -> unit tests
#   - API error codes for bad input          -> API / integration tests

A test drive, not a parts inspection

Unit tests inspect each part on the bench; an E2E test takes the assembled car for a drive around the block. You need both, but you cannot afford to drive every route.

Quick check: Which is the best use of an E2E test?

• Checking that a CSS class name is spelled correctly
• Checking every branch of a tax calculation function
• Checking that a user can complete checkout from cart to confirmation
• Checking the return type of a helper function