record & download tone.js

const recorder = new Tone.Recorder();
const synth = new Tone.Synth().connect(recorder);

// start recording
recorder.start();

// generate a few notes
synth.triggerAttackRelease("C3", 0.5);
synth.triggerAttackRelease("C4", 0.5, "+1");
synth.triggerAttackRelease("C5", 0.5, "+2");
// wait for the notes to end and stop the recording
setTimeout(async () => {
    // the recorded audio is returned as a blob
    const recording = new Blob(await recorder.stop(), {type:'audio/mpeg-3'});
    // download the recording by creating an anchor element and blob url
    const url = URL.createObjectURL(recording);
    const anchor = document.createElement("a");
    anchor.download = "recording.mp3";
    anchor.href = url;
    anchor.click();
}, 4000);