JavaScript merge sort in 30 lines that's only 10% slower than native in V8 - http://jsperf.com/merge-sort/3

merge-sort.js

if (!Array.prototype.mergeSort) {
  Array.prototype.mergeSort = function(predicate) {
    if (this.length < 2) return this

    var middle = Math.round(this.length / 2),
        left = this.slice(0, middle).mergeSort(predicate),
        right = this.slice(middle, this.length).mergeSort(predicate),
        merged = [], leftCursor = 0, rightCursor = 0,
        predicate = predicate || function(a, b) { return a <= b ? -1 : 1 }

    while (leftCursor < left.length && rightCursor < right.length) {
      if (predicate(left[leftCursor], right[rightCursor]) <= 0) {
        merged.push(left[leftCursor])
        leftCursor++
      } else {
        merged.push(right[rightCursor])
        rightCursor++
      }
    }

    if (leftCursor < left.length) {
      merged.splice.apply(merged, left.slice(leftCursor))
    }
    if (rightCursor < right.length) {
      merged.splice.apply(merged, right.slice(rightCursor))
    }

    return merged
  }
}