Frequent Posters Sorting/Filtering by Date in JavaScript

Question

I submitted this exercise for a potential job. I really was hoping to get some meaningful feedback, so I can improve. Instead, they gave me a one line response: "showed a limited understanding of data management techniques and dom interaction." Not sure why they said dom interaction as this has nothing to do with the DOM. Is this really that bad, I know i repeated a bit of code (sorting by date), and my variable naming is not great. But what else? How would you have structured this differently? Would love to know your suggestions so I can improve and build and make the most of this.

Here are the Instructions:

Q: given an array of objects with the format of:

{

title: '',

body: '',

created: Date.toISOString(),

user: {

    name: '',

    id: #

}

}

with efficiency in mind, write a function that returns an array of objects in the format of:

{

name: '',

last_post_title: ''

}

containing the 30 most frequent posters of the last 3 months, sorted by the last time they posted.

Here is my code:

<!DOCTYPE html>
<html>
<head>
    <title>Code Exercise</title>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>

<body>
    <script type="text/javascript">

        // using posts array as test array (only returning the top 2 posters since dataset is small)
        posts = [
        {
            title: 'Waiting 1000 Years for your SO',
            body: '',
            created: '2015-04-05',
            user: {
                    name: 'Rory',
                    id: 0
                }
        },
        {
            title: 'Better Regeneration',
            body: '',
            created: '2014-08-19',
            user: {
                    name: 'The Doctor',
                    id: 1
                }
        },
        {
            title: 'The Craziest Alien in All of Time!',
            body: '',
            created: '2014-08-09',
            user: {
                    name: 'The Doctor',
                    id: 2
                }
        },
                    {
            title: 'The Metaphysical aspects of the Tardis',
            body: '',
            created: '2014-08-27',
            user: {
                    name: 'The Doctor',
                    id: 3
                }
        },
                    {
            title: 'I am not so Impossible, Really',
            body: '',
            created: '2014-08-11',
            user: {
                    name: 'Clara',
                    id: 4
                }
        },
                    {
            title: 'The Bright Side of Being Shattered through Time',
            body: '',
            created: '2014-08-29',
            user: {
                    name: 'Clara',
                    id: 5
                }
        },           
                    {
            title: 'I Saw the Cracks in the Universe, and it was Awesome!',
            body: '',
            created: '2014-08-13',
            user: {
                    name: 'Amy',
                    id: 6
                }
        },
                                {
            title: 'My Imaginary Friend',
            body: '',
            created: '2014-07-15',
            user: {
                    name: 'Amy',
                    id: 7
                }
        }

        ];

        // this function is the answer to the project
        function projectAnswer() {

            // count number of posts, shove into counts array
            var counts = {};
            for (i=0; i < posts.length; i++)  {
                var a = posts[i].user.name;
                counts[a] = 1 + (counts[a] || 0); 
            }

            // massage counts to array of objects
            var newCounts = [];
            for (val in counts) {
                newCounts.push({
                    name: val,
                    numOfPosts: counts[val]    
                }); 
            }

            // sort by number of posts descending
            newCounts.sort(function (a, b) {
                if (a.numOfPosts > b.numOfPosts) {
                  return -1;
                }
                if (a.numOfPosts < b.numOfPosts) {
                  return 1;
                }
                // a must be equal to b
                return 0;
            });

            // Return top 30 posters (returning top 2 so it works with posts array)
            newCounts = newCounts.slice(0, 2);
            // console.log(newCounts);      

            // Use what we know now to pull the top two posters' post objects from posts array, shove into postsSorted array
            var postsSorted = [];
            function sortPosts(param) {
                for (i=0; i < param.length; i++)  {
                    param[i];
                    for (j=0; j < posts.length; j++) {
                        if (param[i].name == posts[j].user.name) {
                            postsSorted.push(posts[j]);
                        }
                    }
                }
            }
            sortPosts(newCounts);

            // sort by created date
            postsSorted.sort(function (a, b) {
                if (a.created > b.created) {
                  return -1;
                }
                if (a.created < b.created) {
                  return 1;
                }
                // a must be equal to b
                return 0;
            });


            function group(postsSorted){

                // get unique values
                var temp = {};
                var out = [];
                var final = [];
                for (var i = 0, l = postsSorted.length; i < l; i++) {
                  var id = postsSorted[i].user.name;
                    if (!temp[id]) {
                        temp[id] = [];
                        temp[id].push(postsSorted[i]);
                        }
                }
                for (var val in temp) {
                  out.push(temp[val]);
                }

                // Push into final array in format we want, array of objects with (name, last_post_title) 
                for (i=0; i < out.length; i++) {
                            final.push({
                                name: out[i][0].user.name,
                                last_post_title: out[i][0].title,
                                // this will list the date for verification purposes
                            });
                        }
                return final;
            }
            var postsSorted = group(postsSorted);

            document.write("<strong>Top 2 Posters' Last Post Sorted by Date: </strong>" +"<br/><br/>" + JSON.stringify(postsSorted));

        };
        projectAnswer();

        </script>

</body>

Community · Accepted Answer · 2017-05-23 11:33:42Z

Without extensively combing through everything, a few things pop up to me:

You use for...in loops in a few places. While they certainly have their place if used correctly, they often aren't, as they can easily produce unexpected results. A quick way to fix this would be to check if the property you're iterating over directly belongs to the object using hasOwnProperty.
In several places, you use i or j as increment variables, but neither are defined anywhere in scope. i and j are global variables!
document.write is seldom-seen because it rarely has a use anywhere in production. You may have more success inserting the result directly into an element in the DOM, which I suspect is where that comment came from.
For loops have 3 parts: for(init, condition, expression) where expression is called every iteration. Every single iteration, a for loop will check if the condition is true, and if so, evaluate the expression and statement. All of your for loops check if somevar < something.length. Looking up the length value for an array takes time, so it is often a good idea to save this result somewhere and use it directly instead. Something like this would perform much faster on a large array:

var i, len;  //No global variables!

for(i = 0, len = something.length; i < len; ++i) {
    //calculate
}

I also pre-increment i rather than post-increment, as post-incrementing creates a copy of the variable, increments, then returns.

The variable a doesn't mean anything to me, but it almost always points to a user.name.

Hopefully this helps!

Anthony · Accepted Answer · 2015-08-13 02:46:55Z

Just as a suggestion, perhaps they were looking for a solution that utilized Array.prototype.map()?

function topPosters(count) {
    var users = [];

    posts.map(function(post){
        var userIndex = users.map(function(user) { return user.name; }).indexOf(post.user.name);

        if(userIndex === -1) {
            var user = {
                        name: post.user.name,
                        posts: 1,
                        lastPost: { 
                                    title: post.title,
                                    date: getDate(post.created)
                                  }
                        };

            users.push(user);
        } else {
            var user = users[userIndex];
            var postDate = getDate(post.created);

            if(postDate > user.lastPost.date) {
                user.lastPost = {
                    title: post.title,
                    date: postDate
                };
            }

            user.posts++;
        }
    });

    return users.sort(function(a,b){
        var x = a.posts;
        var y = b.posts;

        if(x < y) {
            return 1;
        }
        else if(x > y) {
            return -1;
        }
        else {
            if(a.lastPost.date < b.lastPost.date)
                return 1;
            else if(a.lastPost.date > b.lastPost.date)
                return -1;

            return 0;
        }
    }).map(function(user){
        return {
            name: user.name,
            last_post_title: user.lastPost.title
        };
    }).slice(0,count);
}

function getDate(dateStr) {
    var d1s = dateStr.split('-');
    return new Date(parseInt(d1s[0]),parseInt(d1s[1])-1,parseInt(d1s[2]));
}

Hi Tony- Interesting approach. Thanks for sharing. I was curious how ours compared efficiency wise, so I minified both our codes and took the average of 5 tests for load time. Yours loads in 1.98ms and mine in 2.25. So yours is roughly 12% more efficient. Assuming the dataset was 1000x bigger, your code would load .27 seconds faster. Seems like a wash, or are there situations where this could make a difference? — AnotherMike
– AnotherMike, Commented Aug 14, 2015 at 1:26

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