1Internet Explorer is the only browser which really suffers from this in today's world. (Versions 5, 6, and 7 were dog slow. 8 does not show the same degradation.) What's more, IE gets slower and slower the longer your string is.
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3If you have long strings to concatenate then definitely use an array.join technique. (Or some StringBuffer wrapper around this, for readability.) But if your strings are short don't bother.