JS屏蔽Backspace跳转网页

window.onkeydown = function (event) {    var hasPoint = false;    if (event.keyCode + "" == '8') {        switch (event.target.type) {            case "radio":            case "checkbox":                hasPoint = true;                break;            default:                break;        }        if (event.target.tagName != 'INPUT'            && event.target.tagName != 'TEXTAREA') {            hasPoint = true;        }        if (event.target.readOnly == true) {            hasPoint = true;        }        if (hasPoint == true) {            window.event.keyCode = 0;            window.event.returnValue = false;            return false;        }    }}