PayPalCheckoutServer.js

const checkoutNodeJssdk = require('@paypal/checkout-server-sdk');

function paypalEnv() {
  let clientId = process.env.paypal_client_id
  let clientSecret = process.env.paypal_client_secret

  if (process.env.NODE_ENV === 'production') {
    return new checkoutNodeJssdk.core.LiveEnvironment(
      clientId, clientSecret
    )
  } else {
    return new checkoutNodeJssdk.core.SandboxEnvironment(
      clientId, clientSecret
    )
  }
}

const paypal = new checkoutNodeJssdk.core.PayPalHttpClient(paypalEnv());

function handler(req, res) {
    const orderId = req.body.paymentMethodId;
    const captureReq = new checkoutNodeJssdk.orders.OrdersCaptureRequest(orderId);
    captureReq.requestBody({});  
    try {
      const capture = await paypal.execute(captureReq);
      const captureId = capture.result.purchase_units[0]
          .payments.captures[0].id;
      
      // Save captureId to against order in DB

      res.json({message: 'Payment completed'})
    } catch (err) {      
        console.error(err)
        res.status(500).send()
    }
}