webhook-simplify.js

var PORT = process.env.PORT || 8000;
const express = require('express');
const axios = require('axios');

PRODUCT_OPTIONS_ENUM = { COLOR_TITLE: { EN: 'Color' }; // this is more complex object at live

app.post('/product-update', function (req, res, next) {

    const filterByName = (name) => {
      return (item) => {
       return item.name === name;
      }
    };

    const getTags = (data,attr) => {
        return data
            .filter(filterByName(attr))
            .map(i => i.values)
            .reduce((acc, item) => acc.concat(item),[])
            .map( i => attr.toLowerCase() + '-' + i)
    };

    const tags = getTags(req.body.options,PRODUCT_OPTIONS_ENUM.COLOR_TITLE.EN);
   
    axios({
        method: 'put',
        headers : {
            "Authorization" : auth
        },
        url: `https://${username}:${password}@yourshop.myshopify.com/admin/products/${req.body.id}.json`,
        data: {
            "product": {
                "id": req.body.id,
                tags: tags
            }
        }
    })
        .then(result => res.json(result.data))
        .catch(err => res.status(400).json(err));
});

app.listen(PORT, function() {
    console.log("App is running on port " + PORT);
});

simplifyed version of webhooks update flow in shopify