Get/Update for SPListItem at DispForm

sp-contextitemhelper.js

/**
 * sp-contextitemhelper.js
 * Provide functions that manupilates the context list item (displayed now).
 * Assumed for showing/updating hidden fields at the browser.
 *
 * (c) 2017 koohq. Licensed under CC0.
 * https://creativecommons.org/publicdomain/zero/1.0/legalcode
 */
var SPContextItemHelper = (function() {
  var clientCtx = SP.ClientContext.get_current();
  var list = clientCtx.get_web().get_lists().getById(new SP.Guid(_spPageContextInfo.pageListId));
  var item = list.getItemById(parseInt(GetUrlKeyValue('ID'), 10));
 
  function select() {
    clientCtx.load(item);
    clientCtx.executeQueryAsync(function() {
      console.log(item.get_fieldValues());
    }, function(sender, args) {
      console.error('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    });
  }
 
  function update(obj) {
    Object.keys(obj).forEach(function(key) { item.set_item(key, obj[key]); });
    item.update();
 
    clientCtx.executeQueryAsync(function() {
      console.log('Item updated.');
    }, function(sender, args) {
      console.error('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    });
  }
 
  return {
    select: select,
    update: update
  };
})();

sp-contextitemhelper.usage.js

/**
 * sp-contextitemhelper.usage.js
 *
 * (c) 2017 koohq. Licensed under CC0.
 * https://creativecommons.org/publicdomain/zero/1.0/legalcode
 */

// Show the field values on console (contains hidden fields)
ContextItemHelper.select();

// Update the fields values on console (contains hidden fields)
ContextItemHelper.update({ VisibleField: 'xxx', HiddenField: 'yyy' });