JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container"></div>

JavaScript

var dataset  = [ { "field1":"val1", "field2":"val2", "field3":"val3" }, { "field1":"val4", "field2":"val5", "field3":"val6", "field4":"val7" } ] ;

function addHeaders(table, keys) {
  var row = table.insertRow();
  for( var i = 0; i < keys.length; i++ ) {
    var cell = row.insertCell();

    cell.appendChild(document.createTextNode(keys[i]));
  }
}

var max_keys = 0;
var max_idx = 0;
for( var i = 0; i < dataset.length; i++ ) {

  var child = dataset[i];

  cur_keys = Object.keys(child).length;
  if (cur_keys > max_keys) {
      max_keys = cur_keys;
      max_idx = i;
  }

}

var table = document.createElement('table');
addHeaders(table, Object.keys(dataset[max_idx]));

for( var i = 0; i < dataset.length; i++ ) {

  var child = dataset[i];

  var row = table.insertRow();
  Object.keys(child).forEach(function(k) {

    var cell = row.insertCell();
    cell.appendChild(document.createTextNode(child[k]));
  })
}

document.getElementById('container').appendChild(table);