Array.sort无法正常工作

Question

I need the sort function to sort the dates from the earliest date to the latest date. What can I do to fix this in my tasks table?

var tasks = new Array();
var index = 0;

function addTask() {
    var temptask = document.getElementById("taskinfo").value;
    var td = document.getElementById("taskdate").value;
    var tempdate = new Date(td);

    //add array and populate from tempdate and temptask
    //generate html table from 2d javascript array
    tasks[index] = {
        Date: tempdate,
        Task: temptask,
    };  

    index++

    tasks.sort(function(a,b){return new Date(b.Date).getTime() - new Date(a.Date).getTime()});

    var tablecode = "<table class = 'tasktable'>" +
        "<tr>"+
        "<th>Date</th>"+
        "<th>Task</th>"+
        "</tr>";
    for (var i = 0; i < tasks.length; i++) {
        tablecode = tablecode + "<tr>" +
            "<td>" + tasks[i]["Date"].toDateString() + " </td>" +
            "<td>" + tasks[i]["Task"] + " </td>" +
            "</tr>";
    }

    tablecode = tablecode + "</table>";

    document.getElementById("bottomright").innerHTML = tablecode;

    return false;
}

I have tried many different syntax variations and can not get the sort function to sort in descending order

Answer 1

You're subtracting a.Date from b.Date , exactly the reverse of what you want.

Flip those around (and remove the unnecessary new Date() wrappers, although they're not actually breaking anything) and you'll get the correct sort:

 var tasks = [], index = 0; function addTask() { var temptask = document.getElementById("taskinfo").value; var td = document.getElementById("taskdate").value; var tempdate = new Date(td); tasks[index] = { Date: tempdate, Task: temptask, }; index++ tasks.sort(function(a, b) { return a.Date.getTime() - b.Date.getTime() }); var tablecode = "<table class='tasktable'>" + "<tr>" + "<th>Date</th>" + "<th>Task</th>" + "</tr>"; for (var i = 0; i < tasks.length; i++) { tablecode += "<tr>" + "<td>" + tasks[i]["Date"].toDateString() + " </td>" + "<td>" + tasks[i]["Task"] + " </td>" + "</tr>"; } tablecode += "</table>"; document.getElementById("bottomright").innerHTML = tablecode; return false; } document.getElementById('add').addEventListener('click', addTask); 

 <p>Task: <input type="text" id="taskinfo" /></p> <p>Date: <input type="date" id="taskdate" /></p> <button id="add">add</button> <div id="bottomright"></div> 

Answer 2

Since the date is represented as

the number of milliseconds since 1 January, 1970 UTC
( https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date )

the sorting order you are looking for is ascending not descending. Also, as @birdspider already commented, there is no use of creating new Date objects and invoking the getTime() method. They are comparable as they are.

To summarize the above points, try using the following sorting function:

function sortDatesAsc(tempdateA, tempdateB) {
    return tempdateA - tempdateB < 0 ? -1 : (tempdateA > tempdateB ? 1 : 0);
}

tasks.sort(sortDatesAsc);