What’s New in React 16?

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exploring React 16

In this post, we’re going to learn how to create a music player using some of the new features in React 16.

In implementing this music player, we’re going to learn about some of the changes in React 16. There are quite a few changes, so we won’t cover all of them, but we’ll cover the ones that are important and that you can implement today.

The complete source for this post is available on GitHub.

To start the app, download the code, cd into the project directory and type:

npm install
npm start

State in a React Application

All React applications include a property called state that determines how and what components (and any data associated with those components) should be displayed.

Our music player has a state property that contains two important pieces of information: one variable that specifies whether the player is playing music — the playing boolean — and one variable that tracks the state of the current track — the currentTrackIndex variable.

this.state = {
  playing: false,
  currentTrackIndex: 0

What is State?

When we refer to a component’s state, we mean a snapshot of the instance of the component on the page.

React’s components can define their own state, which we’ll use in this post. When we use state in a React component, the component is said to be stateful. A React component can define its own state using a state property for handling stateful components, such as our music player.

As the user clicks the play, pause, next, and previous buttons, and the tracks in the player, our component will update its current state.

Props vs State

For React applications, it’s important to understand the distinction between props and state. Our music player has two state variables that determine the way our application is displayed at a given point in time. The App component is our main component that drives the display of our child components — the Controls component and the TrackList component. In order for these two components to receive information about the state of our application, the App component will pass information down as props to the children components. These props can then be used in the child component to display their pieces of the application correctly. Another important thing to understand is that every time our App component updates, our Controls component and TrackList component will be updated as well, because they rely on information from the App component.


Our Controls component is the first child of our App component. The Controls component is given two props: onClick and playing. The onClick prop allows us to pass down our handleClick function we’ve defined in the App component to the Controls component. When the user clicks one of the buttons in our Controls component, the handleClick function will get called. The playing prop allows the Controls component to know what the current state of the player is so we can properly render the play icon or the pause icon.

Let’s explore how we render our buttons and handle clicks in our Controls component.

In our Controls component we have three important buttons:

  1. The << (previous) button — an arrow icon pointing to the left — which selects the previous track in the list
  2. The play/pause button which plays and pauses the music
  3. The >> (next) button — an arrow icon pointing to the right — which selects the next track in the list.

When each of these buttons is clicked, we call the click handler function that we passed in from the App component. Each of the buttons in our music player application has an id which will aid us in determining how a particular click should be handled.

In the internals of the handleClick function, we use a switch statement that uses the id of the button that was clicked — e.target.id to determine how to handle the action from the button. In the next section, we’ll take a look at what happens in each case of the switch statement.

The play button

When the play button is clicked, we’ll need to update a few parts of our application. We’ll need to switch the play icon to the pause icon. We’ll also need to update the currentTrackIndex if it’s currently set to 0. In order to change these two parts of our application, we’ll call setState, a function available to every React component.

The setState function is available to all React components, and it’s how we update the state of our music player. The first argument in the setState function can either be an object or a function. If we’re not relying on the current state of an application to calculate the next state, using an object as the first argument is a perfectly fine approach and looks like this: this.setState({currentState:'newState'}). In our case, we’re relying on the current state of the application to determine the next state of our application, so we’ll want to use a function. The React documentation indicates why this is important:

React may batch multiple setState() calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

As React 16 turns on more of its features (including asynchronous rendering), this distinction will become more important to understand.

When the play button is clicked and we call setState, we pass in a function, because we’re relying on the current value of the currentTrackIndex state variable. The first argument that’s passed into the function is the previous state of our application, and the second argument is the current props. In our case, we just need the previous state of the application to determine the next state:

case "play":
  this.setState((state, props) => {
    let currentTrackIndex = state.currentTrackIndex;
    if (currentTrackIndex === 0) {
      currentTrackIndex = 1;

Once we’ve set the currentTrackIndex properly based on the previous value of the currentTrackIndex, we then return an object of the values we want to update. In the case of the play button being clicked, we update our playing boolean to true and set the value of the currentTrackIndex:

return {
  playing: true,
  currentTrackIndex: currentTrackIndex

The second argument that’s passed into the setState function is a callback function that’s called after the setState function is completed. When the play button is clicked, and the state of our application is updated, we want to start playing the music. We pass in the this.playAudio function as the second argument to our setState function.


When the playAudio button is called, we reference the audio tag and call the load and play functions available to us via the Web Audio API.


ref to a DOM element

In order to reference the actual audio DOM element to play the audio, we’ll need to use a special attribute available to all React components, the ref attribute. From the React documentation:

When the ref attribute is used on an HTML element, the ref callback receives the underlying DOM element as its argument.

In our situation, we add the ref attribute to our audio DOM element, and that allows us to play the audio for each track:

<audio ref={(audio)=>{this.audioElement = audio}} src={"/songs/"+this.state.currentTrackIndex+".mp3"}/>

The pause button

When the pause button is clicked, we call this.setState and set our playing boolean to false.

case "pause":
  this.setState({ playing: false },this.pauseAudio);

The second argument for our setState function call is our this.pauseAudio function, which references the audio element and calls the pause() function.


The << (previous) button

When the << icon is clicked, the id of the previous button matches the “prev” case of the switch statement, so the code associated with the “prev” case is executed. In the “prev” case, we call this.setState() again with a function like we did for playing and pausing our application. This time, we use the previous value of currentTrackIndex to decrement the value and return an object to set currentTrackIndex to the new value.

case "prev":
  this.setState((state, props) => {
    let currentIndex = state.currentTrackIndex - 1;
    if (currentIndex <= 0) {
      return null;
    } else {
      return { playing:true,currentTrackIndex: currentIndex };

Returning null from setState

One of the new changes in React 16 is that when we return null from a setState function, our application will not be re-rendered. Our track listing has 11 tracks available. If the user continues to click the << button, the currentTrackIndex will decrement until it gets to 0. Once it gets to 0, we no longer want to decrement the currentTrackIndex and we no longer need to re-render our application. We also do the same when our >> icon is clicked. If the currentTrackIndex is equal (or greater than) the number of tracks in our list (11), we return null from setState.

The >> (next) button

When the >> button is called, we have a similar functionality in place as the << button. Each time the user clicks >>, we increment the currentTrackIndex and we check that the currentTrackIndex is not greater than the length of the track list. If it is, we return null in our setState function call.

case "next":
  this.setState((state, props) => {
    let currentIndex = state.currentTrackIndex + 1;
    if (currentIndex > data.tracks.length) {
      return null;
    } else {
      return { playing:true,currentTrackIndex: currentIndex };

Track List

We’ve hard coded the track listing data in a JSON file for ease of understanding the concepts in this post. We import the data from the JSON file at the top, and in our lifecycle method componentDidMount, we set the state of our TrackList component. The state of our TrackList component contains one variable, the tracks variable.

Lifecyle methods componentDidMount and componentDidUpdate

Every React component, in addition to the setState function, also has lifecycle methods available. Our TrackList component uses two of these, componentDidMount and componentDidUpdate. componentDidMount is called when the React component is available in the DOM. In this case, we want to add some data to our component, so calling setState in componentDidMount is the appropriate time to do that.

When our App component updates the currentTrackIndex, the componentDidUpdate method in our TrackList component is triggered, because the TrackList component is getting new data. When the TrackList component gets new data, we want to make sure the currently selected track is in our viewport, so we make some calculations to determine where in the DOM the currently selected track exists and make it appear in the view of the track list container.

componentDidUpdate() {
  if (this.activeTrack) {
    let topOfTrackList = this.trackList.scrollTop;
    let bottomOfTrackList =
      this.trackList.scrollTop + this.trackList.clientHeight;
    let positionOfSelected = this.activeTrack.offsetTop;
    if (
      topOfTrackList > positionOfSelected ||
      bottomOfTrackList < positionOfSelected
    ) {
      this.trackList.scrollTop = positionOfSelected;

Displaying the list of tracks

We use the JavaScript map function to loop over our array of tracks and call a function for each element in the array. The function we call is renderListItem, which contains some logic to determine if the currentTrackIndex is the current element in the array we’re rendering. If it is, we need to make sure the value for the className on the li includes the selected string. This will ensure that the styling for the selected track will be different when compared to the rest of the list.

renderListItem(track, i) {
  let trackClass = this.props.currentTrackIndex === track.id
    ? "selected"
    : "";
  return (
      ref={cur => {
        if (this.props.currentTrackIndex === track.id) {
          this.activeTrack = cur;
      <div className="number">{track.id}</div>
      <div className="title">{track.title}</div>
      <div className="duration">{track.duration}</div>

The li element also contains some other important attributes:

  • key: whenever we have a list, we need to include this attribute so that the list will render properly. For more information on using keys with lists in React, check out this article in the React documentation.

  • className: to make sure the li has the selected class attached to it if it’s the currently selected track.

  • ref: we use the ref attribute to calculate the correct location of the track list container. If the current track is not visible, we calculate the location of the current track and make it visible. We need to access the actual DOM element to make this calculation correctly.

  • onClick: when the user selects a particular track, we call this function, which calls this.props.selectTrackNumber. This function is passed into the TrackList component from our parent App component, just like the click handler for the Controls component. When this function is called, the state of our application is updated, with the currentTrackIndex getting set to the track number the user selected.


Try It Out!

Check out the Codepen example. The album art comes from an album by a band called the Glass Animals. Since we can’t legally stream the “Glass Animals” soundtrack, we’ve picked some royalty-free music to play in its place so we can get the full effect of the music player.

See the Pen React DailyUI – 009 – Music Player by Jack Oliver (@jackoliver) on CodePen.

This post is a part of the React Daily UI post series at Fullstack React, a joint effort between Jack Oliver, Sophia Shoemaker, and the rest of the team at Fullstack React.

Want to dive in deeper into React fundamentals? Check out Fullstack React: The Complete Guide to ReactJS & Friends to learn more.

Sophia ShoemakerSophia Shoemaker
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Sophia Shoemaker has been building websites as a full stack developer since 2009. She's the creator of Shopsifter and the editor of the Fullstack React weekly newsletter.

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