JavaScript Tech Interview Exercise 8: Video Player

You might not know about me that I have conducted tech interviews with over 500 software developers from more than twenty countries with the objective of identifying and hiring the best talent. I have also been in the candidate position, interviewing for positions ranging from junior developer to CTO.

In this series, I am exposing the secrets of JavaScript interviewing. You will get a chance to solve a tech interview exercise every single week. You will not only refresh your JavaScript skills, but above all, you will learn the mindset required for solving interview exercises. We will start with simple tasks, and then transition to complex exercises that you could even use to build your portfolio.

Task: Implement a video player that can play an mp4 video. Add five buttons below the video player:
- 1x, 1.5x, 2x: when clicked, it sets the playback speed to the displayed value on the button
- -30s, +30s when clicked, it offsets the current time of the video by the displayed value

You can use HTML5 tags in the exercise, and you don't have to worry about cross-browser compatibility.

As an example, you can use the video

Make your solution extensible so that it will be easy to add more fully functional playback speed and offset buttons without changing anything in your JavaScript code.


If you have never used the HTML5 video API, it's time to google it. The video markup looks as follows:

We will also need some buttons:

I kept the markup lean, and added some classes that will make it easy to identify each button. As we have to take care of extensibility, it makes sense to reference each button with the same class.

We also used some data attributes to customize the offset. We will read these data in the event handlers.

We will write the event handlers such that the code handles all buttons of the same type in a generic way:

document.querySelectorAll is similar to jQuery's $ function. We pass it a selector, and it returns a DOM node collection. As this collection is an iterable, we can iterate on the elements. We can attach an event listener to each event.

Study the Video API reference to conclude how to change the playback speed and the offset.

Let's start with the playback speed:

Notice how we retrieve the data-speed attribute: contains all the data attributes belonging to a DOM node.

The .js-video video HTML5 element has a playbackRate property. If we set it to a floating point, we can change the playback rate.

We can conclude this exercise with the implementation of the offset buttons:

The video duration is in seconds. We can retrieve the offset from the data-offset attribute via the property The currentTime property of the video contains the place where the video is at currently. After adding the offset to the current time, we have to check if we are still within the boundaries of the video in order to avoid indexing out from the video.

As the currentTime property of the video element is writable, we simply have to assign the new value to it to make the offset work.

Experiment with the solution in this codepen.

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