getUserToken.js

'use strict';
// Require the Box SDK and the fs module
const box = require('box-node-sdk');
const fs = require('fs');

let configFile = fs.readFileSync('config.json');
configFile = JSON.parse(configFile);

// Initialize the SDK with the Box configuration file and create a client that uses the Service Account.
let session = box.getPreconfiguredInstance(configFile);

// Replace with user ID
let userId = "";
session.getAppUserTokens(userId)
    .then((accessTokenObject) => {
        console.log(accessTokenObject);
        console.log(accessTokenObject.accessToken);
    });