So that means JS strings aren't truly immutable in a modern environment (which is fine). The runtime environment is internally using an approach similar to a st

So that means JS strings aren't truly immutable in a modern environment (which is fine). The runtime environment is internally using an approach similar to a string builder, which is a good optimization. I agree that you shouldn't operate on assumptions alone for a decision like whether or not you should use a string builder. That's where prior experience should come in to play to guide your decisions. For instance, I am not a JS developer, so I have no prior experience to inform a decision to use a builder vs concat in JS. I cited that case in particular since the slowness of concatenation was called out in the article, and in some languages it actually does make a huge difference at a very small complexity cost.