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 (typeof foo == 'undefined') {
 // Do something
};

注意，在这种情况下不需要严格比较（！==），因为typeof将始终返回字符串。

我知道检查undefined最可靠的方法是使用void 0。

这与较新和较旧的浏览器都兼容，并且不能像window那样被覆盖。在某些情况下，未定义的浏览器可以被覆盖。

if( myVar === void 0){
    //yup it's undefined
}

如果未定义，它将不等于包含字符“undefined”的字符串，因为该字符串未定义。

您可以检查变量的类型：

if (typeof(something) != "undefined") ...

有时你甚至不必检查类型。如果变量的值在设置时不能求值为false（例如，如果它是一个函数），那么可以只对变量求值。例子：

if (something) {
  something(param);
}

就我个人而言，我总是使用以下方法：

var x;
if( x === undefined) {
    //Do something here
}
else {
   //Do something else here
}

window.undefined属性在所有现代浏览器（JavaScript 1.8.5或更高版本）中都是不可写的。Mozilla文档中：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined，我看到了这一点：使用typeof（）的一个原因是，如果尚未定义变量，它不会抛出错误。

我更喜欢使用

x === undefined 

因为如果x之前没有声明过，它会失败并在我面前爆炸，而不是默默地通过/失败。这提醒我x未声明。我认为JavaScript中使用的所有变量都应该声明。

    var x;
    if (x === undefined) {
        alert ("I am declared, but not defined.")
    };
    if (typeof y === "undefined") {
        alert ("I am not even declared.")
    };

    /* One more thing to understand: typeof ==='undefined' also checks 
       for if a variable is declared, but no value is assigned. In other 
       words, the variable is declared, but not defined. */

    // Will repeat above logic of x for typeof === 'undefined'
    if (x === undefined) {
        alert ("I am declared, but not defined.")
    };
    /* So typeof === 'undefined' works for both, but x === undefined 
       only works for a variable which is at least declared. */

    /* Say if I try using typeof === undefined (not in quotes) for 
       a variable which is not even declared, we will get run a 
       time error. */

    if (z === undefined) {
        alert ("I am neither declared nor defined.")
    };
    // I got this error for z ReferenceError: z is not defined