Sort Array of Dates / Console bug?

Question

I'm confused why the console is showing me a sorted array at both logs. Because at the first point where I'm logging it shouldn't be sorted?

  static reloadAndSortItems() {
    let array = [];
    const items = Store.getStoredItems();

    items.forEach(function (item) {

      // getting the stored date --> back to date object

      let episodeDate = Date.parse(item.episode);
      let parsedEpisode = new Date(episodeDate);


      array.push(parsedEpisode);



    });

    **// should not sorted at this point
    console.log('not sorted', array);**



    let tested = array.sort(function (a, b) {
      return a - b
    });
    **// should be sorted array at this point
    console.log('sorted', tested);**





  }

this is the array that is coming in ( which is out of order) :

["2018-09-13T00:30:00.000Z","2018-09-14T05:25:00.000Z","2018-09-13T00:30:00.000Z","2018-09-11T01:30:00.000Z","2018-09-11T01:30:00.000Z"]

P.S. · Accepted Answer · 2018-09-11 00:30:42Z

1

That's because sort() method mutates the initial array, and also returns a new one, so finally you will get two arrays with the same order of elements:

let arr = [1, 6, 2, 9, 3, 7];
let result = arr.sort((a, b) => a - b);

console.log('Original:', arr);
console.log('Final:', result);

To avoid such behavior, you can create additional array (for example, using map() method, it returns a new array and doesn't mutate the original one), and use it as your initial array:

let arr = [1, 6, 2, 9, 3, 7];
let duplicate = arr.map(d => d);
arr.sort((a, b) => a - b);

console.log('Sorted:', arr);
console.log('Duplicate of the initial array:', duplicate);

edited Sep 11, 2018 at 0:30

answered Sep 11, 2018 at 0:20

P.S.

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Comments

Dacre Denny · Accepted Answer · 2018-09-11 00:30:44Z

The sort() method mutates the array it's called on, so the correct thing to do here is to log to console the array variable, rather than the tested variable:

static reloadAndSortItems() {
    let array = [];
    const items = Store.getStoredItems();

    items.forEach(function (item) {

      // getting the stored date --> back to date object

      let episodeDate = Date.parse(item.episode);
      let parsedEpisode = new Date(episodeDate);    

      array.push(parsedEpisode);    
    });

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

    console.log('sorted', array);    
  }

Alternativly, you could clone the array variable by means of .map(), and then call the .sort() method on that cloned array, like so:

static reloadAndSortItems() {
    let array = [];
    const items = Store.getStoredItems();

    items.forEach(function (item) {

      // getting the stored date --> back to date object

      let episodeDate = Date.parse(item.episode);
      let parsedEpisode = new Date(episodeDate);    

      array.push(parsedEpisode);    
    });

    // Copy/clone the array using map into tested variable
    const tested = array.map(function(item) { return item; });

    // Sort the tested array. Calling sort on tested will leave array unaffected
    tested.sort(function (a, b) {
      return a - b
    });

    console.log('sorted', tested);  // Sorted
    console.log('array', array);    // Unsorted
  }

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