4.js

app.get('/check-coupon', function(req, res, next) {
    stripe.coupons.retrieve(req.body.coupon, function(err, coupon) {
      if (err) return res.json({ valid: false });
      res.json({ valid: true });
    })
  })

check-coupon?coupon=adsf

500 Error: Stripe: I require argument "id", but I got: undefined

step-4.js

  $('.apply button').click(function() {
    $.getJSON('/check-coupon', {
      coupon: $('#coupon').val()
      }, function(data) {
        if (!data.valid) {
          alert('invalid!');
        } else {
          alert('SUCCESS!');
        }
      }
    );
  });