Why array sort is not working as expected?-CodePudding

Can anybody tell me what I am missing here? It doesn't give me the expected output.

I wanted to sort the array in ascending and descending order based on the date but for whatever reason, it only sorts the array in descending order.

let res = [
    ['03-01-2019', 'Test 3'],
    ['03-01-2020', 'Test 1'],
    ['03-01-2021', 'Test 2']
];

const toDate = (dateStr) => {
    const [month, day, year] = dateStr.split("-")
    return new Date(year, month - 1, day)
}

let SortedDefault = res; //default array value, no sorting has been done.
let SortedAscDate = res.sort(function (a, b) { return toDate(a[0]) - toDate(b[0]) }); //sort array in asc order
let SortedDescDate = res.sort(function (a, b) { return toDate(b[0]) - toDate(a[0]) }); //sort array in desc order

document.getElementById("default").innerHTML = SortedDefault;
document.getElementById("asc").innerHTML = SortedAscDate;
document.getElementById("desc").innerHTML = SortedDescDate;

<p id="default"></p>
<p id="asc"></p>
<p id="desc"></p>

I was expecting the following results;

03-01-2019,Test 3,03-01-2020,Test 1,03-01-2021,Test 2 //original, no sorting has been done.
03-01-2019,Test 3,03-01-2020,Test 1,03-01-2021,Test 2 //sort in asc order
03-01-2021,Test 2,03-01-2020,Test 1,03-01-2019,Test 3 //sort in des order

Instead, I got this (all three are descending???);

03-01-2021,Test 2,03-01-2020,Test 1,03-01-2019,Test 3 //original, no sorting has been done (but how can it sorted in des order?) 
03-01-2021,Test 2,03-01-2020,Test 1,03-01-2019,Test 3 //sort in asc, but it has been sort in desc order
03-01-2021,Test 2,03-01-2020,Test 1,03-01-2019,Test 3 //sort in des order, correct result.

CodePudding user response：

The sort method sorts the elements of an array in place.

Use res.slice().sort(...)

CodePudding user response：

.sort() mutates the main array. To overcome you can use a soft copy [...res] to sort like

 let SortedAscDate = [...res].sort(function(a, b) {
  return toDate(a[0]) - toDate(b[0])
})

let res = [
  ['03-01-2019', 'Test 3'],
  ['03-01-2020', 'Test 1'],
  ['03-01-2021', 'Test 2']
];

const toDate = (dateStr) => {
  const [month, day, year] = dateStr.split("-")
  return new Date(year, month - 1, day)
}

let SortedDefault = res; //default array value, no sorting has been done.
let SortedAscDate = [...res].sort(function(a, b) {
  return toDate(a[0]) - toDate(b[0])
}); //sort array in asc order
let SortedDescDate = [...res].sort(function(a, b) {
  return toDate(b[0]) - toDate(a[0])
}); //sort array in desc order

document.getElementById("default").innerHTML = SortedDefault;
document.getElementById("asc").innerHTML = SortedAscDate;
document.getElementById("desc").innerHTML = SortedDescDate;

<p id="default"></p>
<p id="asc"></p>
<p id="desc"></p>