I have an array of a couple thousand strings

['7/21/2011', '7/21/2011', '7/21/2011', '7/20/2011', etc]

I am currently, running this code to group by the string and get the max group value:

var max = 0;
var group = {};
arr.map(function (value) {
  if (group[value]) {
    group[value]++;
  } else {
    group[value] = 1;
  }
  max = Math.max(max, group[value]);
});

Are there any improvements to make this code run faster?

EDIT: The results are in: http://jsperf.com/javascript-array-grouping2

EDIT EDIT: that test was flawed. Mike Samuel's code was the fastest.

6000 entries test -> http://jsperf.com/javascript-array-grouping2

10K entries test -> http://jsperf.com/javascript-array-grouping

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