我找到了这篇文章，《在JavaScript中处理未定义的7个技巧》，它展示了关于未定义的真正有趣的事情如：

undefined的存在是JavaScript的许可性质的结果，允许使用：

未初始化变量不存在的对象财产或方法访问数组元素的越界索引不返回任何内容的函数的调用结果

Use:

要检查属性是否未定义，请执行以下操作：

if (typeof something === "undefined") {
    alert("undefined");
}

要检查属性是否未定义，请执行以下操作：

if (typeof something !== "undefined") {
    alert("not undefined");
}

交叉发布我对相关问题的回答如何在JavaScript中检查“undefined”？。

具体到这个问题，请参阅someObject的测试用例<无论什么>。

说明各种答案结果的一些场景：http://jsfiddle.net/drzaus/UVjM4/

（请注意，在测试中使用var会在作用域包装器中产生差异）

参考代码：

(function(undefined) {
    var definedButNotInitialized;
    definedAndInitialized = 3;
    someObject = {
        firstProp: "1"
        , secondProp: false
        // , undefinedProp not defined
    }
    // var notDefined;

    var tests = [
        'definedButNotInitialized in window',
        'definedAndInitialized in window',
        'someObject.firstProp in window',
        'someObject.secondProp in window',
        'someObject.undefinedProp in window',
        'notDefined in window',

        '"definedButNotInitialized" in window',
        '"definedAndInitialized" in window',
        '"someObject.firstProp" in window',
        '"someObject.secondProp" in window',
        '"someObject.undefinedProp" in window',
        '"notDefined" in window',

        'typeof definedButNotInitialized == "undefined"',
        'typeof definedButNotInitialized === typeof undefined',
        'definedButNotInitialized === undefined',
        '! definedButNotInitialized',
        '!! definedButNotInitialized',

        'typeof definedAndInitialized == "undefined"',
        'typeof definedAndInitialized === typeof undefined',
        'definedAndInitialized === undefined',
        '! definedAndInitialized',
        '!! definedAndInitialized',

        'typeof someObject.firstProp == "undefined"',
        'typeof someObject.firstProp === typeof undefined',
        'someObject.firstProp === undefined',
        '! someObject.firstProp',
        '!! someObject.firstProp',

        'typeof someObject.secondProp == "undefined"',
        'typeof someObject.secondProp === typeof undefined',
        'someObject.secondProp === undefined',
        '! someObject.secondProp',
        '!! someObject.secondProp',

        'typeof someObject.undefinedProp == "undefined"',
        'typeof someObject.undefinedProp === typeof undefined',
        'someObject.undefinedProp === undefined',
        '! someObject.undefinedProp',
        '!! someObject.undefinedProp',

        'typeof notDefined == "undefined"',
        'typeof notDefined === typeof undefined',
        'notDefined === undefined',
        '! notDefined',
        '!! notDefined'
    ];

    var output = document.getElementById('results');
    var result = '';
    for(var t in tests) {
        if( !tests.hasOwnProperty(t) ) continue; // bleh

        try {
            result = eval(tests[t]);
        } catch(ex) {
            result = 'Exception--' + ex;
        }
        console.log(tests[t], result);
        output.innerHTML += "\n" + tests[t] + ": " + result;
    }
})();

结果：

definedButNotInitialized in window: true
definedAndInitialized in window: false
someObject.firstProp in window: false
someObject.secondProp in window: false
someObject.undefinedProp in window: true
notDefined in window: Exception--ReferenceError: notDefined is not defined
"definedButNotInitialized" in window: false
"definedAndInitialized" in window: true
"someObject.firstProp" in window: false
"someObject.secondProp" in window: false
"someObject.undefinedProp" in window: false
"notDefined" in window: false
typeof definedButNotInitialized == "undefined": true
typeof definedButNotInitialized === typeof undefined: true
definedButNotInitialized === undefined: true
! definedButNotInitialized: true
!! definedButNotInitialized: false
typeof definedAndInitialized == "undefined": false
typeof definedAndInitialized === typeof undefined: false
definedAndInitialized === undefined: false
! definedAndInitialized: false
!! definedAndInitialized: true
typeof someObject.firstProp == "undefined": false
typeof someObject.firstProp === typeof undefined: false
someObject.firstProp === undefined: false
! someObject.firstProp: false
!! someObject.firstProp: true
typeof someObject.secondProp == "undefined": false
typeof someObject.secondProp === typeof undefined: false
someObject.secondProp === undefined: false
! someObject.secondProp: true
!! someObject.secondProp: false
typeof someObject.undefinedProp == "undefined": true
typeof someObject.undefinedProp === typeof undefined: true
someObject.undefinedProp === undefined: true
! someObject.undefinedProp: true
!! someObject.undefinedProp: false
typeof notDefined == "undefined": true
typeof notDefined === typeof undefined: true
notDefined === undefined: Exception--ReferenceError: notDefined is not defined
! notDefined: Exception--ReferenceError: notDefined is not defined
!! notDefined: Exception--ReferenceError: notDefined is not defined

if (somevariable == undefined) {
  alert('the variable is not defined!');
}

您也可以将其转换为函数，如下所示：

function isset(varname){
  return(typeof(window[varname]) != 'undefined');
}

在《探索JavaScript中的空和未定义的深渊》一文中，我看到Undercore.js等框架使用了这个函数：

function isUndefined(obj){
    return obj === void 0;
}

我使用if（this.variable）来测试它是否被定义。一个简单的if（变量），在前面的答案中推荐，对我来说失败了。

事实证明，只有当变量是某个对象obj.someField的字段时，它才能工作，以检查它是否在字典中定义。但我们可以使用this或window作为字典对象，因为任何变量都是当前窗口中的字段，正如我所理解的那样。因此，这里有一个测试：

如果（this.abc）警报（“已定义”）；其他的警报（“未定义”）；abc=“abc”；如果（this.abc）警报（“已定义”）；其他的警报（“未定义”）；

它首先检测到变量abc未定义，并在初始化后定义。