javascript - 如何对具有空值的数组进行排序

与该主题相关的问题很多，但我找不到适合我的案例的正确解决方案。

var arr = [a, b, null, d, null]

并使用以下逻辑对该数组进行排序

return function(a,b){

    if(a === null){
      return 1;
    }
    else if(b === null){
      return -1;
    }
    else if(a === b){
      return 0;
    }
    else if(ascending) {
      return a < b ? -1 : 1;
    }
    else if(!ascending) {
      return a < b ? 1 : -1;
    }
  };

我得到以下输出

Ascending : [a, b,  d, null,null]

Descending : [d, b,  a, null,null]

Expected : [null, null,d, b,  a]

我究竟做错了什么？

最佳答案

function getSort (ascending) {
  // if ascending, `null` will be pushed towards the end of the array by returning 1
  var nullPosition = ascending ? 1 : -1
  return function (a, b) {
    // if a is null, push it towards whichever end null elements should end up
    if (a == null) return nullPosition

    // Note: at this point, a is non-null (previous if statement handled that case).
    //
    // If b is null, it must therefore be placed closer to whichever end the null
    // elements should end up on. If ascending, null elements are pulled towards
    // the right end of the array. If descending, null elements are pulled towards
    // the left.
    //
    // Therefore, we return -nullPosition. If ascending, this is -1, meaning a comes
    // before b; if descending, this is 1, meaning a comes after b. This is
    // clearly the correct behavior, since ascending will push b, which is null,
    // towards the end of the array (with -1) and descending will push b towards
    // the beginning of the array.
    if (b == null) return -nullPosition

    // OTHERWISE, both elements are non-null, so sort normally.
    // if a < b AND
    //     if ascending, a comes first, so return -1 == -nullPosition
    //     if descending, a comes after, so return -nullPosition == -(-1) == 1
    if (a < b) return -nullPosition

    // return the opposite of the previous condition
    if (a > b) return nullPosition

    // return 0 if both elements are equal
    return 0
  }
}

function write (arr) { arr.forEach(function (d) { document.write(d + "<br>")})}

var toSort = ['a', 'b', null, 'd', null]

var sortA = getSort(true)
var sortD = getSort(false)

document.write("<br>ASCENDING<br>")
write(toSort.sort(sortA))

document.write("<br>DESCENDING<br>")
write(toSort.sort(sortD))