Sort array by date gives unexpected results

Question

This will sound as an easy problem but I spent my Sunday trying to figure out what's wrong with my implementation described below so I am posting it to SO as a last resort.

I have a javascript application that receives data structures from a server. The server side sends the data unsorted for performance reasons.

Here is a snippet of the the javascript code receiving data:

    var seriesRawDataArray = ko.observableArray();     ...     analyticscontext.series(seriesRawDataArray).done(function () {         renderSeries();     }); 

The analyticscontext module queries the data using ajax:

function series(seriesData) {     return $.ajax({         url: "/api/analytics/series",         type: "GET",         success: function (data) {             return seriesData(data);         }     }); } 

The renderSeries performs a sort on the data before rendering it:

    // Sort the data by date using moment.js     seriesRawDataArray.sort(function (left, right) {         var leftDate = moment.utc(left.timeStamp);         var rightDate = moment.utc(right.timeStamp);         var diff = leftDate.diff(rightDate);         return diff > 0;     }); 

PROBLEM

Here is a data sample I receive from my server:

Notice the unsorted items at the end. the seriesRawDataArray.sort seem to have no effect on the original array which does not get sorted no matter what I change in the sorting method. The output is always:

Notice the unsorted elements here. The libraries I am using and the data is definitely not the problem as this jsfiddle works just fine! Is there something wrong with this code?

Answer 1

short answer

You should return the difference between the two dates, not a boolean:

// sort the data by date using moment.js seriesRawDataArray.sort(function (left, right) {     return moment.utc(left.timeStamp).diff(moment.utc(right.timeStamp)) }); 

why

Array.prototype.sort expects a negative, zero, or positive value to be returned. Generally, you'll write a sort function like this:

yourArray.sort(function (a, b) {     if (a < b) {            // a comes first         return -1     } else if (b < a) {     // b comes first         return 1     } else {                // equal, so order is irrelevant         return 0            // note: sort is not necessarily stable in JS     } }) 

^{The anonymous function passed to sort serves as the comparator for the native implementation of the sort function.}

However, your negative value doesn't have to be -1 and your positive value doesn't have to be +1. Therefore, when sorting numbers, you can instead use the shortcut:

yourArray.sort(function (a, b) {     return a - b }) 

In JavaScript, subtracting two dates coerces them both into numbers, which is why we could use return moment.utc(left.timeStamp).diff(moment.utc(right.timeStamp))

^{(instead of direct subtraction -, this method uses moment.prototype.diff from the moment.js library)}

However, in your code, you returned diff > 0, which can be either true or false. Because of type coercion, JavScript will read true as 1 and false as 0. This means your sort function will never return -1. Therefore, your elements will not be sorted correctly.