Share variables between index.html and app.js [node.js]

//index.html
var audioCtx = new AudioContext();
var mySampleRate = audioCtx.sampleRate;

//app.js
var fileWriter = new wav.FileWriter(n + '.wav', {
    channels: 1,
    sampleRate: mySampleRate,

    bitDepth: 16
  });

I want to share de mySampleRate [value] of index.html with the node.js serve. How can I do this?