Drawing sinewave in JS canvas does not do anything

Question

I wrote the following to draw a sound wave from AudioBuffer but what I get is a canvas with a straight horizontal line:

const { audioContext, analyser } = this.getAudioContext();
const source = audioContext.createBufferSource();
source.buffer = audioBuffer;
source.connect(audioContext.destination);

const canvas = document.getElementById('canvas');
const canvasCtx = canvas.getContext("2d");

let sinewaveDataArray = new Uint8Array(analyser.fftSize);

const drawSinewave = function() {
    analyser.getByteTimeDomainData(sinewaveDataArray);
    requestAnimationFrame(drawSinewave);

    canvasCtx.fillStyle = 'white';
    canvasCtx.fillRect(0, 0, canvas.width, canvas.height);
    canvasCtx.lineWidth = 2;
    canvasCtx.strokeStyle = "black";
    canvasCtx.beginPath();

    const sliceWidth = canvas.width * 1.0 / analyser.fftSize;
    let x = 0;

    for(let i = 0; i < analyser.fftSize; i++) {
        const v = sinewaveDataArray[i] / 128.0; // byte / 2 || 256 / 2
        const y = v * canvas.height / 2;

        if(i === 0) {
            canvasCtx.moveTo(x, y);
        } else {
            canvasCtx.lineTo(x, y);
        }
        x += sliceWidth;
    }

    canvasCtx.lineTo(canvas.width, canvas.height / 2);
    canvasCtx.stroke();
};

source.start();

drawSinewave();

getAudioContext =  () => {
        AudioContext = window.AudioContext || window.webkitAudioContext;
        const audioContext = new AudioContext();
        const analyser = audioContext.createAnalyser();

        return { audioContext, analyser };
    };

The end result I am looking for is something like the first image clip here: https://meyda.js.org/

Any idea what I am missing?

Answer 1

I think two little changes are necessary to make it work.

source.start() needs to be executed in response to a user event. A simple button with a click handler can be used to achieve that. This is necessary to deal with the autoplay policy which is present in some browsers.

aReferenceToAButton.addEventListener('click', async () => {
    await audioContext.resume();
    source.start();
});

Last but not least you need to pipe the signal through the AnalyserNode.

source
    .connect(analyser)
    .connect(audioContext.destination);

I hope this helps.