排序算法的JavaScript实现

let compare = (n1, n2) => n1 - n2;

let bubbleSort = (arr, cmp = compare) => {
  for (let i = 0; i < arr.length; i++) {
    for (let j = i; j > 0; j--) {
      if (cmp(arr[j], arr[j - 1]) < 0) {
        [arr[j], arr[j - 1]] = [arr[j - 1], arr[j]];
      }
    }
  }
  return arr;
};

let insertionSort = (arr) => {
    for (let i = 0; i < a.length; i++) {
        let toCmp = arr[i];
        for (let j = i; j > 0 && toCmp < a[j - 1]; j--)
            arr[j] = a[j - 1];
        arr[j] = toCmp;
    }
    return arr;
}

var selectionSort = function (arr) {
  let i,m,j;
  for (i = -1; ++i < a.length;) {
    for (m = j = i; ++j < a.length;) {
      if (arr[m] > arr[j]) m = j;
    }
    [arr[m], arr[i]] = [arr[i], arr[m]];
 }
 return arr;
}

let mergeSort = (arr) => {

  if (arr.length < 2) return arr;

  let middle = parseInt(arr.length / 2),
  left = arr.slice(0, middle),
  right = arr.slice(middle);

  return merge(mergeSort(left), mergeSort(right));
}

let merge = (left, right) => {
  let result = [];

  while (left.length && right.length) {
    left[0] <= right[0] ?
    result.push(left.shift()) :
    result.push(right.shift());
  }

  while (left.length) result.push(left.shift());
  while (right.length) result.push(right.shift());

  return result;
}

let quicksort = function(arr) {
  if(arr.length <= 1) return arr;

  let pivot = Math.floor((arr.length -1)/2);
  let val = arr[pivot], less = [], more = [];

  arr.splice(pivot, 1);
  arr.forEach(function(e,i,a){
    e < val ? less.push(e) : more.push(e);
  });
  return (quicksort(less)).concat([val],quicksort(more))
}