Add times with reduce

This is the 18th project of Wes Bos's JS30 series. To see the whole 30 part series, click here Today we'll be using map and reduce to add time of individual videos find the total time of a playlist.

Video -

Codepen -

The HTML structure we have is something similar to what there would be in a youtube playlist -

<ul class="videos">
  <li data-time="5:43">
    Video 1
  </li>
  <li data-time="2:33">
    Video 2
  </li>
  ...
</ul>

The <ul> represents the playlist, each <li> is a video item. The data-time attribute holds the time of the video. What we need to do is find the total time of the playlist, i.e. add the individual times (represented as a string) of each element.

1. Extract all the times from the <li> into array
2. Convert each string in the array to seconds
3. Add all the seconds
4. Display the total time in "hh:mm:ss" format

Extract times from items

We use Array.from to create an array object from the NodeList object document.querySelectorAll gives us. Then we map the array to the data-time value of each element to get an array of times in strings.

const items = Array.from(document.querySelectorAll('.videos li'))
const times = items.map(item => item.dataset.time)

Convert string array into seconds array

We use map to convert each string into an integer which represents the time of the video in seconds.

const seconds = times.map(time => {
    [min, sec] = time.split(':').map(parseFloat)
    return (min*60) + sec
})

If we have something like '4:30', it is split into ["4", "30"] and we convert it into numbers with parseFloat(). We capture the minutes and seconds in min, sec by ES6 destructuring.

Add all the seconds

We use reduce to add all the time in seconds to get the total time in seconds.

let totalSec = seconds.reduce((sum, time) => sum + time)

Display the total time

We have to split the time in sec to hours, minutes and seconds. totalSec is the time we have in seconds, assume it's equal to 125. First let's shave off the whole minutes from the seconds - totalSec % 60 gives us the reminder of seconds once we have divided by 60 (we use 60 since one min has 60 sec). 125 % 60 will give us 5. Now totalMin is the number of whole minutes we have, that is (125-5) / 60 viz 2 minutes. We repeat a similar process for hours - shave off any extra minutes and divide by 60 to get the whole hours. Try it out with a bigger number (greater than 3600, since there are 3600 secs in an hour).

let secs = totalSec % 60
let totalMin = (totalSec - secs) / 60

let mins = totalMin % 60
let hours = (totalMin - mins) / 60

console.log(hours + ':' + mins + ':' + secs)

This completes our exercise. Final codepen -