Snippet to add an audio compressor to all videos on the current page

compressor.js

function addCompressor(el) {
    var AudioContext = window.AudioContext || window.webkitAudioContext;
    var context = new AudioContext();
    var source = context.createMediaElementSource(el);
    var compressor = context.createDynamicsCompressor();
    var makeupGain = context.createGain();
    
    // Set compressor params here
    compressor.threshold.value = -40;
    compressor.knee.value = 40;
    compressor.ratio.value = 5;
    compressor.attack.value = 0;
    compressor.release.value = 0.25;
    // Makeup gain formula from http://music-dsp.music.columbia.narkive.com/3BVi9E9D/computing-compressor-automatic-makeup-gain
    makeupGain.gain.value = 0.5 * Math.abs(compressor.threshold.value) / compressor.ratio.value;
    
    source.connect(compressor);
    compressor.connect(makeupGain);
    makeupGain.connect(context.destination);
    
    console.log(`Applying ${makeupGain.gain.value}db makeup gain`);
}

document.querySelectorAll("video").forEach(addCompressor);