Read JSON endpoint, create TABLE HTML

json2html.js

//ES5 version
fetch(uri)
    .then(response => response.json())
    .then(json => {
            // Create HTML TABLE, for a challenge refactor this to one .reduceRight call
            let headers = [], maxid = 0;// maximum Primary Key value read in this DB response
            // process rows first because the headers info is needed in the THEAD
            let TBODY = json.map(function (row) {
                headers = Object.keys(row);
                let TRdata = '';// stuff all data values as data-attributes on the TR tag
                let TR = headers.map(function (columnName) {
                    insertRow()
                    let value = row[columnName];
                    if (columnName.indexOf('_') === 0) {// its a database primary _key
                        if (value > maxid) maxid = value;
                    }
                    let data = ' data-' + columnName + '="' + value + '"';
                    TRdata += data;
                    return '<TD data-column="' + columnName + '" ' + data + '>' + value + '</TD>';
                }).join('');
                return '<TR' + TRdata + '>' + TR + '</TR>';
            }).join('');

            this.innerHTML = '<TABLE data-query="' + uri + '" data-maxid=' + maxid + '><THEAD><TR>'
                + headers.map(function (columnName) {
                    return '<TH data-column="' + columnName + '">' + columnName + '</TH>';
                }).join('') + '</TR></THEAD>' + TBODY + '</TABLE>';
        }
    );