Get the server's gateway ip address in a node script.

gateway.js

const util = require('util')
const execFile = util.promisify(require('child_process').execFile); // node 8+

const getGatewayIp = () =>
  new Promise((resolve, reject) => {
    execFile('/sbin/ip', ["route"])
      .then(result => {
        if (result.stdout) {
          const ip_pattern = /\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b/;
          const ip = result.stdout.match(ip_pattern)[0];
          resolve(ip);
        }
        if (result.stderr) {
          console.log(`stderr: ${result.stderr}`);
          reject(stderr);
	}
      })
      .catch(e => reject(e));
  })

getGatewayIp()
  .then(result => console.log(`IP address is ${result}`))
  .catch(e => console.log(`Exception: ${e}`));