请帮我找出其中的错误:
var username = document.getElementById("username_input").value;
var password = document.getElementById("password_input").value;
var password_confirm = document.getElementById("password_confirm").value;
xhr.open('POST', 'header/onsubmit_check.php');
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xhr.send("username=" + username + "&password"= + password +"&password_confirm=" + password_confirm);