[Octane]

var input;
try {
    input = document.createElement('<input type="text" name="searchfield[]" />');
}
catch (error) {
    input = document.createElement("input");
    input.setAttibute("type", "text");
    input.setAttibute("name", "searchfield[]");
}

или так:

function parseHTML(htmlCode) {
    var tmp = document.createElement("div");
    tmp.innerHTML = htmlCode;
    return tmp.firstChild;
}

var input = parseHTML('<input type="text" name="searchfield[]" />');