Calls callback for each item in this in ascending order ( 0 to length-1 ). It passes the return value of callback for the i-1 th item as the previous parameter for the i th item. Returns the result from the last call to callback . If initialValue is not specified, callback will first be called on this[1] with previous set to this[0] . The Array passed to callback is the this of the call to reduce .

Example: var join = function(prev, cur) { return prev + cur; } console.log(['b', 'c'].reduce(join, 'a')); Run Results: