• When a test is especially short or simple compared to the application code it tests, lean toward writing the test first.
  • When the desired behavior isn’t yet crystal clear, lean toward writing the application code first, then write a test to codify the result.
  • Because security is a top priority, err on the side of writing tests of the security model first.
  • Whenever a bug is found, write a test to reproduce it and protect against regressions, then write the application code to fix it.
  • Lean against writing tests for code (such as detailed HTML structure) likely to change in the future.
  • Write tests before refactoring code, focusing on testing error-prone code that’s especially likely to break.

— Michael Hartl, Ruby on Rails Tutorial

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