BAU: Test-driven development

source/partials/_svg-tdd-red-green-refactor.html.erb

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source/standards/test-driven-development.html.md.erb

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+---
+title: Test-driven development (TDD)
+last_reviewed_on: 2024-10-21
+review_in: 6 months
+---
+# <%= current_page.data.title %>
+
+GDS advocates for agile software development practices because we believe that flexibility and responsiveness-to-change are key to building software that meets the needs of our users.
+
+Test-driven development (TDD) is a core agile software development practice which aims to build flexibility and correctness into production software.
+
+The process starts with a set of desired behaviours for a piece of code. This list could include happy-path behaviours, error cases, and interaction points with other components.
+
+The inner-loop of TDD is also commonly referred to as "Red-Green-Refactor"; red to represent a failing test, green to represent all passing tests, and refactoring only when all tests are passing.
+
+<%= partial 'partials/svg-tdd-red-green-refactor' %>
+
+### Red (a failing test)
+Turn exactly one item on the list into an actual, concrete, runnable test. This test should fail when you run it.
+
+### Green (all tests passing)
+Change the code to make all tests, including the new test, pass. If you discover that you are missing a behaviour or test, add them to the list
+
+
+### Refactor (iterative and incremental design)
+
+Improve the design and implementation of the code by removing duplication, utilising well known patterns and structures.
+
+These steps are repeated until your set of behaviours is exhausted.
+
+## Why we recommend it
+
+The advantage of writing the tests before the implementation is that the behaviour of the code you will write is defined first, rather than implementing the change and retroactively applying tests.
+
+A test-first approach gives you quick feedback on the design of your code - if you code is becoming difficult to test or has many dependencies, these are signals that you likely need to refactor.
+
+> For each desired change, make the change easy (warning: this may be hard), then make the easy change - Kent Beck
+
+Test-driven development naturally lends itself to our recommended approach to [breaking down work](/standards/breaking-down-work.html). If you are careful about introducing feature toggles so that code can be "dark launched" into production environments, test-driven development can give you a robust and reliable iterative workflow.
+
+For example, if you are defining a new component, you could first define the interface, integrate a "do nothing" or noop version of that component into the live codebase, and then iterate on that implementation or another implementation safely.
+
+## What if we can't do TDD?
+
+The most valuable output of the TDD process is not the code itself but the tight feedback loops that allow you to reflect and the iterative design decisions that you make along the way.
+
+If doing what Kent Beck calls [canon TDD][canon-tdd] is not possible, for whatever reason, that's okay as long as you have another mechanism for realising the fast feedback and incremental delivery advantages that TDD does provide.
+
+[canon-tdd]: https://tidyfirst.substack.com/p/canon-tdd