Javascript 或门（| |）将false排序为未定义

我有这样一个代码：

var IFS =
    document.isFullScreen || 
    document.webkitIsFullScreen || 
    document.mozIsFullScreen || 
    document.msIsFullScreen;

var IFS =
    (typeof document.isFullScreen != "undefined") ? document.isFullScreen :
    (typeof document.webkitIsFullScreen != "undefined") ? document.webkitIsFullScreen :
    (typeof document.mozIsFullScreen != "undefined") ? document.mozIsFullScreen :
    (typeof document.msIsFullScreen != "undefined") ? document.msIsFullScreen : 
    false;

var arr = ['isFullScreen', 'webkitIsFullScreen', 'mozIsFullScreen', 'msIsFullScreen'];
var IFS = false;
for (var i = 0; i < arr.length; i++) {
    if (arr[i] in document) {
        IFS = document[arr[i]];
        break;
    }
}

确切的问题是，

|

门将

假

排序为

未定义

，因为

是全屏

是布尔值。

---

如何修复这个？

<代码>未定义是一个FalSee值，意味着<代码> < < /代码>运算符将转换为与“代码> false < /代码>相同。 如果只想找到未定义的第一个值，可以尝试以下方法：

var IFS =
    document.isFullScreen || 
    document.webkitIsFullScreen || 
    document.mozIsFullScreen || 
    document.msIsFullScreen;

var IFS =
    (typeof document.isFullScreen != "undefined") ? document.isFullScreen :
    (typeof document.webkitIsFullScreen != "undefined") ? document.webkitIsFullScreen :
    (typeof document.mozIsFullScreen != "undefined") ? document.mozIsFullScreen :
    (typeof document.msIsFullScreen != "undefined") ? document.msIsFullScreen : 
    false;

var arr = ['isFullScreen', 'webkitIsFullScreen', 'mozIsFullScreen', 'msIsFullScreen'];
var IFS = false;
for (var i = 0; i < arr.length; i++) {
    if (arr[i] in document) {
        IFS = document[arr[i]];
        break;
    }
}

或作为：

或者可以使用数组，如下所示：

var IFS =
    document.isFullScreen || 
    document.webkitIsFullScreen || 
    document.mozIsFullScreen || 
    document.msIsFullScreen;

var IFS =
    (typeof document.isFullScreen != "undefined") ? document.isFullScreen :
    (typeof document.webkitIsFullScreen != "undefined") ? document.webkitIsFullScreen :
    (typeof document.mozIsFullScreen != "undefined") ? document.mozIsFullScreen :
    (typeof document.msIsFullScreen != "undefined") ? document.msIsFullScreen : 
    false;

var arr = ['isFullScreen', 'webkitIsFullScreen', 'mozIsFullScreen', 'msIsFullScreen'];
var IFS = false;
for (var i = 0; i < arr.length; i++) {
    if (arr[i] in document) {
        IFS = document[arr[i]];
        break;
    }
}

---

如果每个元素都未定义，则IFS将未定义。由于undefined是一个伪值，因此您仍然可以使用以下条件语句：

var IFS =
    document.isFullScreen || 
    document.webkitIsFullScreen || 
    document.mozIsFullScreen || 
    document.msIsFullScreen;

if(!IFS){
    console.log('not full screen');
}

如果您仍然希望在其他变量未定义的情况下保持

false

值，则可以使用以下方法：

var IFS =
    document.isFullScreen || 
    document.webkitIsFullScreen || 
    document.mozIsFullScreen || 
    document.msIsFullScreen || 
    false;

---

文档中的“isFullScreen”可能更简洁。@NiettheDarkAbsol好的观点，等一下。如果没有所有的

类型的…==“undefined”

混乱。我会给它一个向上投票，对不起，但谁做了向下投票？你需要否定中的

操作的结果，使其与第一个示例等效（或几乎是无论如何）。呵呵，很好，我没有想到的第二个代码，谢谢！！()