V8 custom formulas

Make a field automatically be a mathematical result. Return 'total' as JSON key with value 'price' * 'tax_rate'. price and tax_rate are stored in a database, total is not.

var lodash = require('lodash.min.js');
 
lodash._.each(event.response.content.resource, function (record) {
 
    if (!record.tax) {
 
        throw "Tax is required!";
    }
 
    if (!record.price) {
 
        throw "Price is required!";
    }
 
    record.total = (parseFloat(record.tax) * parseFloat(record.price) / 100) + parseFloat(record.price);
});