Issue #3432151 by Tom Konda, longwave: [jQuery 4] ajax.js and jquery.form.js...

        // Sanity check for browser support (object expected).

        // When using iFrame uploads, responses must be returned as a string.

        if (typeof response === 'string') {

          response = $.parseJSON(response);

          response = JSON.parse(response);

        }

        // Prior to invoking the response's commands, verify that they can be

				return (doc && doc.documentElement && doc.documentElement.nodeName !== 'parsererror') ? doc : null;

			};

			var parseJSON = $.parseJSON || function(s) {

				/* jslint evil:true */

				return window['eval']('(' + s + ')');			// eslint-disable-line dot-notation

			};

			var httpData = function(xhr, type, s) { // mostly lifted from jq1.4.4

				}

				if (typeof data === 'string') {

					if ((type === 'json' || !type) && ct.indexOf('json') >= 0) {

						data = parseJSON(data);

						data = JSON.parse(data);

					} else if ((type === 'script' || !type) && ct.indexOf('javascript') >= 0) {

						$.globalEval(data);

					}