greasemonkey - GM_getValue 未定義エラー

Question

グリースモンキー スクリプトで、GM 値: ユーザー名とパスワードが設定されているかどうかを確認したいのですが、次のコードを試すとエラーが返されます。

TypeError: GM_getValue(...) is undefined    
...f (GM_getValue ("username").length == 0 + GM_getValue ("password").length == 0 )

コード：

if (GM_getValue ("username").length == 0 + GM_getValue ("password").length == 0 ){
var username = $('input[name=username]');
var password = $('input[name=password]');

//Username en Password in Firefox zetten met GM_setValue
$(".button").click(function(){

GM_setValue ("username", username.val() );
GM_setValue ("password", password.val() );

});
}

Answer 1

First, I'm not sure if you can check the length of a function's return value directly like that. Second, you definitely shouldn't be adding booleans like that either, you need the boolean-AND operator && instead of +. Try something like this:

var username = GM_getValue("username");
var password = GM_getValue("password");
if ((username.length == 0) && (password.length == 0)) {
    username = $('input[name=username]').val();
    password = $('input[name=password]').val();
}
$(".button").click(function(){
    GM_setValue ("username", username);
    GM_setValue ("password", password);
});