if the code is a crystallisation of their business idea then using it again is equivalent to re-using their business idea and i think that is highly un-ethical. if you are talking about reusing some CSS templates and changing the colors - meh.
personally, i err on the side of caution and never take work with me from one company to another - often i can condense 6 months of learn-by-doing into a couple of weeks of do-over since it is the overall direction and knowing what is definitely wrong that saves time.
/note: i'm a scientist not a programmer but i think the same situations apply
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if the code is a crystallisation of their business idea then using it again is equivalent to re-using their business idea and i think that is highly un-ethical. if you are talking about reusing some CSS templates and changing the colors - meh. personally, i err on the side of caution and never take work with me from one company to another - often i can condense 6 months of learn-by-doing into a couple of weeks of do-over since it is the overall direction and knowing what is definitely wrong that saves time. /note: i'm a scientist not a programmer but i think the same situations apply