HTMLOutputElement.labels

var labelElements = output.labels;

<label id="label1" for="test">Label 1</label>
<output id="test">Output</output>
<label id="label2" for="test">Label 2</label>

window.addEventListener("DOMContentLoaded", function() {
  const output = document.getElementById("test");
  for(var i = 0; i < output.labels.length; i++) {
    console.log(output.labels[i].textContent); // "Label 1" and "Label 2"
  }
});