A Guide To Audio Visualization With JavaScript And GSAP (Part 2)

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Jhey makes awesome things for awesome people! He’s worked on the web for 10+ years and is currently a Developer Relations Engineer @ Google. He’s … More about Jhey ↬

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Quick summary ↬ What started as a case study turned into a guide to visualizing audio with JavaScript. Although the output demos are in React, Jhey Tompkins isn’t going to dwell on the React side of things too much. The underlying techniques work with or without React.

Last week in Part 1, I explained how the idea about how to record audio input from users and then moved on to the visualization. After all, without any visualization, any type of audio recording UI isn’t very engaging, is it? Today, we’ll be diving into more details in terms of adding features and any sort of extra touches you like!

We’ll be covering the following:

Please note that in order to see the demos in action, you’ll need to open and test directly them on the CodePen website.

Pausing A Recording

Pausing a recording doesn’t take much code at all.

// Pause a recorder
recorder.pause()
// Resume a recording
recorder.resume()

In fact, the trickiest part about integrating recording is designing your UI. Once you’ve got a UI design, it’ll likely be more about the changes you need for that.

Also, pausing a recording doesn’t pause our animation. So we need to make sure we stop that too. We only want to add new bars whilst we are recording. To determine what state the recorder is in, we can use the state property mentioned earlier. Here’s our updated toggle functionality:

const RECORDING = recorder.state === 'recording'
// Pause or resume recorder based on state.
TOGGLE.style.setProperty('--active', RECORDING ? 0 : 1)
timeline[RECORDING ? 'pause' : 'play']()
recorder[RECORDING ? 'pause' : 'resume']()

And here’s how we can determine whether to add new bars in the reporter or not.

REPORT = () => {
  if (recorder && recorder.state === 'recording') {

Challenge: Could we also remove the report function from gsap.ticker for extra performance? Try it out.

For our demo, we’ve changed it so the record button becomes a pause button. And once a recording has begun, a stop button appears. This will need some extra code to handle that state. React is a good fit for this but we can lean into the recorder.state value.

See the Pen [15. Pausing a Recording](http://codepen.io/smashingmag/pen/BamgQEP) by Jhey.

See the Pen 15. Pausing a Recording by Jhey.

Padding Out The Visuals

Next, we need to pad out our visuals. What do we mean by that? Well, we go from an empty canvas to bars streaming across. It’s quite a contrast and it would be nice to have the canvas filled with zero volume bars on start. There is no reason we can’t do this either based on how we are generating our bars. Let’s start by creating a padding function, padTimeline:

// Move BAR_DURATION out of scope so it’s a shared variable.
const BAR_DURATION =
  CANVAS.width / ((CONFIG.barWidth + CONFIG.barGap) * CONFIG.fps)

const padTimeline = () => {
  // Doesn’t matter if we have more bars than width. We will shift them over to the correct spot
  const padCount = Math.floor(CANVAS.width / CONFIG.barWidth)

  for (let p = 0; p < padCount; p++) {
    const BAR = {
      x: CANVAS.width + CONFIG.barWidth / 2,
      // Note the volume is 0
      size: gsap.utils.mapRange(
        0,
        100,
        CANVAS.height * CONFIG.barMinHeight,
        CANVAS.height * CONFIG.barMaxHeight
      )(volume),
    }
    // Add to bars Array
    BARS.push(BAR)
    // Add the bar animation to the timeline
    // The actual pixels per second is (1 / fps * shift) * fps
    // if we have 50fps, the bar needs to have moved bar width before the next comes in
    // 1/50 = 4 === 50 * 4 = 200
    timeline.to(
      BAR,
      {
        x: `-=${CANVAS.width + CONFIG.barWidth}`,
        ease: 'none',
        duration: BAR_DURATION,
      },
      BARS.length * (1 / CONFIG.fps)
    )
    }
  // Sets the timeline to the correct spot for being added to
  timeline.totalTime(timeline.totalDuration() - BAR_DURATION)
}

The trick here is to add new bars and then set the playhead of the timeline to where the bars fill the canvas. At the point of padding the timeline, we know that we only have padding bars so totalDuration can be used.

timeline.totalTime(timeline.totalDuration() - BAR_DURATION)

Notice how that functionality is very like what we do inside the REPORT function? We have a good opportunity to refactor here. Let’s create a new function named addBar. This adds a new bar based on the passed volume.

const addBar = (volume = 0) => {
  const BAR = {
    x: CANVAS.width + CONFIG.barWidth / 2,
    size: gsap.utils.mapRange(
      0,
      100,
      CANVAS.height * CONFIG.barMinHeight,
      CANVAS.height * CONFIG.barMaxHeight
    )(volume),
  }
  BARS.push(BAR)
  timeline.to(
    BAR,
    {
      x: `-=${CANVAS.width + CONFIG.barWidth}`,
      ease: 'none',
      duration: BAR_DURATION,
    },
    BARS.length * (1 / CONFIG.fps)
  )
}

Now our padTimeline and REPORT functions can make use of this:

const padTimeline = () => {
  const padCount = Math.floor(CANVAS.width / CONFIG.barWidth)
  for (let p = 0; p < padCount; p++) {
    addBar()
  }
  timeline.totalTime(timeline.totalDuration() - BAR_DURATION)
}

REPORT = () => {
  if (recorder && recorder.state === 'recording') {
    ANALYSER.getByteFrequencyData(DATA_ARR)
    const VOLUME = Math.floor((Math.max(...DATA_ARR) / 255) * 100)
    addBar(VOLUME)
  }
  if (recorder || visualizing) {
    drawBars()
  }
}

Now, on load, we can do an initial rendering by invoking padTimeline followed by drawBars.

padTimeline()
drawBars()

Putting it all together and that’s another neat feature!

See the Pen [16. Padding out the Timeline](http://codepen.io/smashingmag/pen/OJOebYE) by Jhey.

See the Pen 16. Padding out the Timeline by Jhey.

How We Finish

Do you want to pull the component down or do a rewind, maybe a rollout? How does this affect performance? A rollout is easier. But a rewind is trickier and might have perf hits.

More after jump! Continue reading below ↓

Finishing The Recording

You can finish up your recording any way you like. You could stop the animation and leave it there. Or, if we stop the animation we could roll back the animation to the start. This is often used in various UI/UX designs. And the GSAP API gives us a neat way to do this. Instead of clearing our timeline on stop, we can move this into where we start a recording to reset the timeline. But, once we’ve finished a recording, let’s keep the animation around so we can use it.

STOP.addEventListener('click', () => {
  if (recorder) recorder.stop()
  AUDIO_CONTEXT.close()
  // Pause the timeline
  timeline.pause()
  // Animate the playhead back to the START_POINT
  gsap.to(timeline, {
    totalTime: START_POINT,
    onComplete: () => {
      gsap.ticker.remove(REPORT)
    }
  })
})

In this code, we tween the totalTime back to where we set the playhead in padTimeline. That means we needed to create a variable for sharing that.

let START_POINT

And we can set that within padTimeline.

const padTimeline = () => {
  const padCount = Math.floor(CANVAS.width / CONFIG.barWidth)
  for (let p = 0; p < padCount; p++) {
    addBar()
  }
  START_POINT = timeline.totalDuration() - BAR_DURATION
  // Sets the timeline to the correct spot for being added to
  timeline.totalTime(START_POINT)
}

We can clear the timeline inside the RECORD function when we start a recording:

// Reset the timeline
timeline.clear()

And this gives us what is becoming a pretty neat audio visualizer:

See the Pen [17. Rewinding on Stop](http://codepen.io/smashingmag/pen/LYOKbKW) by Jhey.

See the Pen 17. Rewinding on Stop by Jhey.

Scrubbing The Values On Playback

Now we’ve got our recording, we can play it back with the <audio> element. But, we’d like to sync our visualization with the recording playback. With GSAP’s API, this is far easier than you might expect.

const SCRUB = (time = 0, trackTime = 0) => {
  gsap.to(timeline, {
    totalTime: time,
    onComplete: () => {
      AUDIO.currentTime = trackTime
      gsap.ticker.remove(REPORT)
    },
  })
}
const UPDATE = e => {
  switch (e.type) {
    case 'play':
      timeline.totalTime(AUDIO.currentTime + START_POINT)
      timeline.play()
      gsap.ticker.add(REPORT)
      break
    case 'seeking':
    case 'seeked':
      timeline.totalTime(AUDIO.currentTime + START_POINT)
      break
    case 'pause':
      timeline.pause()
      break
    case 'ended':
      timeline.pause()
      SCRUB(START_POINT)
      break
  }
}

// Set up AUDIO scrubbing
['play', 'seeking', 'seeked', 'pause', 'ended']
  .forEach(event => AUDIO.addEventListener(event, UPDATE))

We’ve refactored the functionality that we use when stopping to scrub the timeline. And then it’s a case of listening for different events on the <audio> element. Each event requires updating the timeline playhead. We can add and remove REPORT to the ticker based on when we play and stop audio. But, this does have an edge case. If you seek after the audio has “ended”, the visualization won’t render updates. And that’s because we remove REPORT from the ticker in SCRUB. You could opt to not remove REPORT at all until a new recording begins or you move to another state in your app. It’s a matter of monitoring performance and what feels right.

The fun part here though is that if you make a recording, you can scrub the visualization when you seek 😎

See the Pen [18. Syncing with Playback](http://codepen.io/smashingmag/pen/qBVzRaj) by Jhey.

See the Pen 18. Syncing with Playback by Jhey.

At this point, you know everything you need to know. But, if you want to learn about some extra things, keep reading.

Audio Playback From Other Sources

One thing we haven’t looked at is how you visualize audio from a source other than an input device. For example, an mp3 file. And this brings up an interesting challenge or problem to think about.

Let’s consider a demo where we have an audio file URL and we want to visualize it with our visualization. We can explicitly set our AUDIO element’s src before visualizing.

AUDIO.src = 'http://assets.codepen.io/605876/lobo-loco-spencer-bluegrass-blues.mp3'
// NOTE:: This is required in some circumstances due to CORS
AUDIO.crossOrigin = 'anonymous'

We no longer need to think about setting up the recorder or using the controls to trigger it. As we have an audio element, we can set the visualization to hook into the source direct.

const ANALYSE = stream => {
  if (AUDIO_CONTEXT) return
  AUDIO_CONTEXT = new AudioContext()
  ANALYSER = AUDIO_CONTEXT.createAnalyser()
  ANALYSER.fftSize = CONFIG.fft
  const DATA_ARR = new Uint8Array(ANALYSER.frequencyBinCount)
  SOURCE = AUDIO_CONTEXT.createMediaElementSource(AUDIO)
  const GAIN_NODE = AUDIO_CONTEXT.createGain()
  GAIN_NODE.value = 0.5
  GAIN_NODE.connect(AUDIO_CONTEXT.destination)
  SOURCE.connect(GAIN_NODE)
  SOURCE.connect(ANALYSER)

  // Reset the bars and pad them out...
  if (BARS && BARS.length > 0) {
    BARS.length = 0
    padTimeline()
  }

  REPORT = () => {
    if (!AUDIO.paused || !played) {
      ANALYSER.getByteFrequencyData(DATA_ARR)
      const VOLUME = Math.floor((Math.max(...DATA_ARR) / 255) * 100)
      addBar(VOLUME)
      drawBars()  
    }
  }
  gsap.ticker.add(REPORT)
}

By doing this we can connect our AudioContext to the audio element. We do this using createMediaElementSource(AUDIO) instead of createMediaStreamSource(stream). And then the audio elements’ controls will trigger data getting passed to the analyzer. In fact, we only need to create the AudioContext once. Because once we’ve played the audio track, we aren’t working with a different audio track after. Hence, the return if AUDIO_CONTEXT exists.

if (AUDIO_CONTEXT) return

One other thing to note here. Because we’re hooking up the audio element to an AudioContext, we need to create a gain node. This gain node allows us to hear the audio track.

SOURCE = AUDIO_CONTEXT.createMediaElementSource(AUDIO)
const GAIN_NODE = AUDIO_CONTEXT.createGain()
GAIN_NODE.value = 0.5
GAIN_NODE.connect(AUDIO_CONTEXT.destination)
SOURCE.connect(GAIN_NODE)
SOURCE.connect(ANALYSER)

Things do change a little in how we process events on the audio element. In fact, for this example, when we’ve finished the audio track, we can remove REPORT from the ticker. But, we add drawBars to the ticker. This is so if we play the track again or seek, etc. we don’t need to process the audio again. This is like how we handled playback of the visualization with the recorder.

This update happens inside the SCRUB function and you can also see a new played variable. We can use this to determine whether we’ve processed the whole audio track.

const SCRUB = (time = 0, trackTime = 0) => {
  gsap.to(timeline, {
    totalTime: time,
    onComplete: () => {
      AUDIO.currentTime = trackTime
      if (!played) {
        played = true
        gsap.ticker.remove(REPORT)
        gsap.ticker.add(drawBars) 
      }
    },
  })
}

Why not add and remove drawBars from the ticker based on what we are doing with the audio element? We could do this. We could look at gsap.ticker._listeners and determine if drawBars was already used or not. We may choose to add and remove when playing and pausing. And then we could also add and remove when seeking and finishing seeking. The trick would be making sure we don’t add to the ticker too much when “seeking”. And this would be where to check if drawBars was already part of the ticker. This is of course dependent on performance though. Is that optimization going to be worth the minimal performance gain? It comes down to what exactly your app needs to do. For this demo, once the audio gets processed, we are switching out the ticker function. That’s because we don’t need to process the audio again. And leaving drawBars running in the ticker shows no performance hit.

const UPDATE = e => {
  switch (e.type) {
    case 'play':
      if (!played) ANALYSE()
      timeline.totalTime(AUDIO.currentTime + START_POINT)
      timeline.play()
      break
    case 'seeking':
    case 'seeked':
      timeline.totalTime(AUDIO.currentTime + START_POINT)
      break 
    case 'pause':
      timeline.pause()
      break
    case 'ended':
      timeline.pause()
      SCRUB(START_POINT)
      break
  }
}

Our switch statement is much the same but we instead only ANALYSE if we haven’t played the track.

And this gives us the following demo:

See the Pen [19. Processing Audio Files](http://codepen.io/smashingmag/pen/rNYEjWe) by Jhey.

See the Pen 19. Processing Audio Files by Jhey.

Challenge: Could you extend this demo to support different tracks? Try extending the demo to accept different audio tracks. Maybe a user can select from dropdown or input a URL.

This demo leads to an interesting problem that arose when working on “Record a Call” for Kent C. Dodds. It’s not one I’d needed to deal with before. In the demo above, start playing the audio and seek forwards in the track before it finishes playing. Seeking forwards breaks the visualization because we are skipping ahead of time. And that means we are skipping processing certain parts of the audio.

How can you resolve this? It’s an interesting problem. You want to build the animation timeline before you play audio. But, to build it, you need to play through the audio first. Could you disable “seeking” until you’ve played through once? You could. At this point, you might start drifting into the world of custom audio players. Definitely out of scope for this article. In a real-world scenario, you may be able to put server-side processing in place. This might give you a way to get the audio data ahead of time before playing it.

For Kent’s “Record a Call”, we can take a different approach. We are processing the audio as it’s recorded. And each bar gets represented by a number. If we create an Array of numbers representing the bars, we already have the data to build the animation. When a recording gets submitted, the data can go with it. Then when we make a request for audio, we can get that data too and build the visualization before playback.

We can use the addBar function we defined earlier whilst looping over the audio data Array.

// Given an audio data Array example
const AUDIO_DATA = [100, 85, 43, 12, 36, 0, 0, 0, 200, 220, 130]

const buildViz = DATA => {
  DATA.forEach(bar => addBar(bar))
}

buildViz(AUDIO_DATA)

Building our visualizations without processing the audio again is a great performance win.

Consider this extended demo of our recording demo. Each recording gets stored in localStorage. And we can load a recording to play it. But, instead of processing the audio to play it, we build a new bars animation and set the audio element src.

Note: You need to scroll down to see stored recordings in the <details> and <summary> element.

See the Pen [20. Saved Recordings ✨](http://codepen.io/smashingmag/pen/KKyjaaP) by Jhey.

See the Pen 20. Saved Recordings ✨ by Jhey.

What needs to happen here to store and playback recordings? Well, it doesn’t take much as we have the bulk of functionality in place. And as we’ve refactored things into mini utility functions, this makes things easier.

Let’s start with how we are going to store the recordings in localStorage. On page load, we are going to hydrate a variable from localStorage. If there is nothing to hydrate with, we can instantiate the variable with a default value.

const INITIAL_VALUE = { recordings: []}
const KEY = 'recordings'
const RECORDINGS = window.localStorage.getItem(KEY)
  ? JSON.parse(window.localStorage.getItem(KEY))
  : INITIAL_VALUE

Now. It’s worth noting that this guide isn’t about building a polished app or experience. It’s giving you the tools you need to go off and make it your own. I’m saying this because some of the UX, you might want to put in place in a different way.

To save a recording, we can trigger a save in the ondataavailable method we’ve been using.

recorder.ondataavailable = (event) => {
  // All the other handling code
  // save the recording
  if (confirm('Save Recording?')) {
    saveRecording()
  }
}

The process of saving a recording requires a little “trick”. We need to convert our AudioBlob into a String. That way, we can save it to localStorage. To do this, we use the FileReader API to convert the AudioBlob to a data URL. Once we have that, we can create a new recording object and persist it to localStorage.

const saveRecording = async () => {
  const reader = new FileReader()
  reader.onload = e => {
    const audioSafe = e.target.result
    const timestamp = new Date()
    RECORDINGS.recordings = [
      ...RECORDINGS.recordings,
      {
        audioBlob: audioSafe,
        metadata: METADATA,
        name: timestamp.toUTCString(),
        id: timestamp.getTime(),
      },
    ]
    window.localStorage.setItem(KEY, JSON.stringify(RECORDINGS))
    renderRecordings()
    alert('Recording Saved')  
  }
  await reader.readAsDataURL(AUDIO_BLOB)
}

You could create whatever type of format you like here. For ease, I’m using the time as an id. The metadata field is the Array we use to build our animation. The timestamp field is being used like a “name”. But, you could do something like name it based on the number of recordings. Then you could update the UI to allow users to rename the recording. Or you could even do it through the save step with window.prompt.

In fact, this demo uses the window.prompt UX so you can see how that would work.

See the Pen [21. Prompt for Recording Name 🚀](http://codepen.io/smashingmag/pen/oNorBwp) by Jhey.

See the Pen 21. Prompt for Recording Name 🚀 by Jhey.

You may be wondering what renderRecordings does. Well, as we aren’t using a framework, we need to update the UI ourselves. We call this function on load and every time we save or delete a recording.

The idea is that if we have recordings, we loop over them and create list items to append to our recordings list. If we don’t have any recordings, we are showing a message to the user.

For each recording, we create two buttons. One for playing the recording, and another for deleting the recording.

const renderRecordings = () => {
  RECORDINGS_LIST.innerHTML = ''
  if (RECORDINGS.recordings.length > 0) {
    RECORDINGS_MESSAGE.style.display = 'none'
    RECORDINGS.recordings.reverse().forEach(recording => {
      const LI = document.createElement('li')
      LI.className = 'recordings__recording'
      LI.innerHTML = `<span>${recording.name}</span>`
      const BTN = document.createElement('button')
      BTN.className = 'recordings__play recordings__control'
      BTN.setAttribute('data-recording', recording.id)
      BTN.title = 'Play Recording'
      BTN.innerHTML = SVGIconMarkup
      LI.appendChild(BTN)
      const DEL = document.createElement('button')
      DEL.setAttribute('data-recording', recording.id)
      DEL.className = 'recordings__delete recordings__control'
      DEL.title = 'Delete Recording'
      DEL.innerHTML = SVGIconMarkup
      LI.appendChild(DEL)
      BTN.addEventListener('click', playRecording)
      DEL.addEventListener('click', deleteRecording)
      RECORDINGS_LIST.appendChild(LI)
    })
  } else {
    RECORDINGS_MESSAGE.style.display = 'block'
  }
}

Playing a recording means setting the AUDIO element src and generating the visualization. Before playing a recording or when we delete a recording, we reset the state of the UI with a reset function.

const reset = () => {
  AUDIO.src = null
  BARS.length = 0
  gsap.ticker.remove(REPORT)
  REPORT = null
  timeline.clear()
  padTimeline()
  drawBars()
}

const playRecording = (e) => {
  const idToPlay = parseInt(e.currentTarget.getAttribute('data-recording'), 10)
  reset()
  const RECORDING = RECORDINGS.recordings.filter(recording => recording.id === idToPlay)[0]
  RECORDING.metadata.forEach(bar => addBar(bar))
  REPORT = drawBars
  AUDIO.src = RECORDING.audioBlob
  AUDIO.play()
}

The actual method of playback and showing the visualization comes down to four lines.

RECORDING.metadata.forEach(bar => addBar(bar))
REPORT = drawBars
AUDIO.src = RECORDING.audioBlob
AUDIO.play()
  1. Loop over the metadata Array to build the timeline.
  2. Set the REPORT function to drawBars.
  3. Set the AUDIO src.
  4. Play the audio which in turn triggers the animation timeline to play.

Challenge: Can you spot any edge cases in the UX? Any issues that could arise? What if we are recording and then choose to play a recording? Could we disable controls when we are in recording mode?

To delete a recording, we use the same reset method but we set a new value in localStorage for our recordings. Once we’ve done that, we need to renderRecordings to show the updates.

const deleteRecording = (e) => {
  if (confirm('Delete Recording?')) {
    const idToDelete = parseInt(e.currentTarget.getAttribute('data-recording'), 10)
    RECORDINGS.recordings = [...RECORDINGS.recordings.filter(recording => recording.id !== idToDelete)]
    window.localStorage.setItem(KEY, JSON.stringify(RECORDINGS))
    reset()
    renderRecordings()    
  }
}

At this stage, we have a functional voice recording app using localStorage. It makes for an interesting start point that you could take and add new features to and improve the UX. For example, how about making it possible for users to download their recordings? Or what if different users could have different themes for their visualization? You could store colors, speeds, etc. against recordings. Then it would be a case of updating the canvas properties and catering for changes in the timeline build. For “Record a Call”, we supported different canvas colors based on the team a user was part of.

This demo supports downloading tracks in the .ogg format.

See the Pen [22. Downloadable Recordings 🚀](http://codepen.io/smashingmag/pen/bGYPgqJ) by Jhey.

See the Pen 22. Downloadable Recordings 🚀 by Jhey.

But you could take this app in various directions. Here are some ideas to think about:

  • Reskin the app with a different “look and feel”
  • Support different playback speeds
  • Create different visualization styles. For example, how might you record the metadata for a waveform type visualization?
  • Displaying the recordings count to the user
  • Improve the UX catching edge cases such as the recording to playback scenario from earlier.
  • Allow users to choose their audio input device
  • Take your visualizations 3D with something like ThreeJS
  • Limit the recording time. This would be vital in a real-world app. You would want to limit the size of the data getting sent to the server. It would also enforce recordings to be concise.
  • Currently, downloading would only work in .ogg format. We can’t encode the recording to mp3 in the browser. But you could use serverless with ffmpeg to convert the audio to .mp3 for the user and return it.

Turning This Into A React Application

Well. If you’ve got this far, you have all the fundamentals you need to go off and have fun making audio recording apps. But, I did mention at the top of the article, we used React on the project. As our demos have got more complex and we’ve introduced “state”, using a framework makes sense. We aren’t going to go deep into building the app out with React but we can touch on how to approach it. If you’re new to React, check out this “Getting Started Guide” that will get you in a good place.

The main problem we face when switching over to React land is thinking about how we break things up. There isn’t a right or wrong. And then that introduces the problem of how we pass data around via props, etc. For this app, it’s not too tricky. We could have a component for the visualization, the audio playback, and recordings. And then we may opt to wrap them all inside one parent component.

For passing data around and accessing things in the DOM, React.useRef plays an important part. This is “a” React version of the app we’ve built.

See the Pen [23. Taking it to React Land 🚀](http://codepen.io/smashingmag/pen/ZEadLyW) by Jhey.

See the Pen 23. Taking it to React Land 🚀 by Jhey.

As stated before, there are different ways to achieve the same goal and we won’t dig into everything. But, we can highlight some of the decisions you may have to make or think about.

For the most part, the functional logic remains the same. But, we can use refs to keep track of certain things. And it’s often the case we need to pass these refs in props to the different components.

return (
  <>
    <AudioVisualization
      start={start}
      recording={recording}
      recorder={recorder}
      timeline={timeline}
      drawRef={draw}
      metadata={metadata}
      src={src}
    />
    <RecorderControls
      onRecord={onRecord}
      recording={recording}
      paused={paused}
      onStop={onStop}
    />
    <RecorderPlayback
      src={src}
      timeline={timeline}
      start={start}
      draw={draw}
      audioRef={audioRef}
      scrub={scrub}
    />
    <Recordings
      recordings={recordings}
      onDownload={onDownload}
      onDelete={onDelete}
      onPlay={onPlay}
    />
  </>
)

For example, consider how we are passing the timeline around in a prop. This is a ref for a GreenSock timeline.

const timeline = React.useRef(gsap.timeline())

And this is because some of the components need access to the visualization timeline. But, we could approach this a different way. The alternative would be to pass in event handling as props and have access to the timeline in the scope. Each way would work. But, each way has trade-offs.

Because we’re working in “React” land, we can shift some of our code to be “Reactive”. The clue is in the name, I guess. 😅 For example, instead of trying to pad the timeline and draw things from the parent. We can make the canvas component react to audio src changes. By using React.useEffect, we can re-build the timeline based on the metadata available:

React.useEffect(() => {
  barsRef.current.length = 0
  padTimeline()
  drawRef.current = DRAW
  DRAW()
  if (src === null) {
    metadata.current.length = 0      
  } else if (src && metadata.current.length) {
    metadata.current.forEach(bar => addBar(bar))
    gsap.ticker.add(drawRef.current)
  }
}, [src])

The last part that would be good to mention is how we persist recordings to localStorage with React. For this, we are using a custom hook that we built before in our “Getting Started” guide.

const usePersistentState = (key, initialValue) => {
  const [state, setState] = React.useState(
    window.localStorage.getItem(key)
      ? JSON.parse(window.localStorage.getItem(key))
      : initialValue
  )
  React.useEffect(() => {
    // Stringify so we can read it back
    window.localStorage.setItem(key, JSON.stringify(state))
  }, [key, state])
  return [state, setState]
}

This is neat because we can use it the same as React.useState and we get abstracted away from persisting logic.

// Deleting a recording
setRecordings({
  recordings: [
    ...recordings.filter(recording => recording.id !== idToDelete),
  ],
})
// Saving a recording
const audioSafe = e.target.result
const timestamp = new Date()
const name = prompt('Recording name?')
setRecordings({
  recordings: [
    ...recordings,
    {
      audioBlob: audioSafe,
      metadata: metadata.current,
      name: name || timestamp.toUTCString(),
      id: timestamp.getTime(),
    },
  ],
})

I’d recommend digging into some of the React code and having a play if you’re interested. Some things work a little differently in React land. Could you extend the app and make the visualizer support different visual effects? For example, how about passing colors via props for the fill style?

That’s It!

Wow. You’ve made it to the end! This was a long one.

What started as a case study turned into a guide to visualizing audio with JavaScript. We’ve covered a lot here. But, now you have the fundamentals to go forth and make audio visualizations as I did for Kent.

Last but not least, here’s one that visualizes a waveform using @react-three/fiber:

See the Pen [24. Going to 3D React Land 🚀](http://codepen.io/smashingmag/pen/oNoredR) by Jhey.

See the Pen 24. Going to 3D React Land 🚀 by Jhey.

That’s ReactJS, ThreeJS and GreenSock all working together! 💪

There’s so much to go off and explore with this one. I’d love to see where you take the demo app or what you can do with it!

As always, if you have any questions, you know where to find me.

Stay Awesome! ʕ •ᴥ•ʔ

P.S. There is a CodePen Collection containing all the demos seen in the articles along with some bonus ones. 🚀

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