checkout.js

// Compressed version of quick start

<script src="https://www.paypalobjects.com/api/checkout.js"></script>
<div id="paypal-button"></div>
<script>
  paypal.Button.render({
    env: 'sandbox',
    client: { sandbox: 'demo_sandbox_client_id' },
    payment: function(data, actions) {
      return actions.payment.create({
        transactions: [{
          amount: { total: '0.01', currency: 'USD' }
        }]
      });
    },
    onAuthorize: function(data, actions) {
      return actions.payment.execute().then(function () {
        window.alert('Thanks for your purchase!');
      });
    }
  }, '#paypal-button');
</script>


// Or the super-compressed code-golf version
// (uses some javascript patterns not available in older browsers though...)

<script src="https://www.paypalobjects.com/api/checkout.js"></script>
<script>
  paypal.Button.render({
    env: 'sandbox',
    client: { sandbox: 'demo_sandbox_client_id' },
    payment: (data, actions) => actions.payment.create({
       transactions: [{ amount: { total: '0.01', currency: 'USD' } }]
    }),
    onAuthorize: (data, actions) => actions.payment.execute().then(
      () => window.alert('Thanks for your purchase!')),
  }, document.body);
</script>