Avoiding rewrites is probably the biggest lesson I've learned. We face the urge to rewrite our front-end applications per sprint almost but I think everyone underestimates how much of the existing codebase can't just be lifted into another codebase especially when dependencies differ.
Another important thing I've learned is that the team should take some time to familiarize with the framework and tools in use. Lack of knowledge of the existing tools leads to weeks of work being spent reinventing the wheel only to find that a simple function provided by the framework provides significantly better performance than our in-house solution.
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