XRRenderState.layers

const xrSession = navigator.xr.requestSession("immersive-ar", {
  optionalFeatures: ["layers"]
}

function onXRSessionStarted(xrSession) {
  const glCanvas = document.createElement("canvas");
  const gl = glCanvas.getContext("webgl", { xrCompatible: true });
  const xrGlBinding = new XRWebGLBinding(xrSession, gl);
  const projectionLayer = new XRWebGLLayer(xrSession, gl);
  const quadLayer = xrGlBinding.createQuadLayer({ pixelWidth: 1024, pixelHeight: 1024 });

  xrSession.updateRenderState({
    layers: [projectionLayer, quadLayer]
  });

  xrSession.renderState.layers; // [projectionLayer, quadLayer]
}