Mountainside is a borough in Union County, New Jersey, United States. New Jersey Monthly magazine ranked Mountainside as its 16th best place to live in its 2008 rankings of the "Best Places To Live" in New Jersey, as well as eighth in the 2010 list. The borough is bordered to the north by Summit, to the east by Springfield Township, to the south by Westfield, and to the west by Berkeley Heights and Scotch Plains.
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