// x has not been defined before
if (typeof x === 'undefined') { // Evaluates to true without errors.
   // These statements execute.
}

if (x === undefined) { // Throws a ReferenceError

}

您可以使用typeof，如下所示：

if (typeof something != "undefined") {
    // ...
}

    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 

// x has not been defined before
if (typeof x === 'undefined') { // Evaluates to true without errors.
   // These statements execute.
}

if (x === undefined) { // Throws a ReferenceError

}

我使用它作为函数参数，并在函数执行时排除它，这样我就得到了未定义的“真实”。尽管它确实需要您将代码放在函数中。我在阅读jQuery源代码时发现了这一点。

undefined = 2;

(function (undefined) {
   console.log(undefined); // prints out undefined
   // and for comparison:
   if (undeclaredvar === undefined) console.log("it works!")
})()

当然，你可以使用typeof。但无论如何，我的所有代码通常都在一个包含函数中，所以使用这个方法可能会在这里和那里节省一些字节。

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);
}