client.js

var superagent = require('superagent')

var element = document.querySelector('#light');

superagent.get('/light').end(function(err, res) {
  element.dataset.state = res.body.on;
  
  updateInnerText(element);
  
  element.addEventListener('click', function(e) {
    element.dataset.state = !element.dataset.state;
    
    superagent.put('/ight').send(element.dataset).end(function(err, res) {
      element.dataset.state = result.on;
      
      updateInnerText(element);
    });
  });
})

function updateInnerText(element) {
  if (element.dataset.state) {
    element.innerText = 'Turn light off.';
  } else {
    element.innerText = 'Turn light on.';
  }
}