如何在JavaScript中检查“undefined”？

我知道的检查 undefined 最可靠的方法是使用 void 0。

这适用于新旧浏览器，且不像某些情况下可以被覆盖的 window.undefined。

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

因为其他回答都没能帮到我，所以我建议这样做。在Internet Explorer 8中，这对我有用：

if (typeof variable_name.value === 'undefined') {
    // variable_name is undefined
}

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

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

}

与@Thomas Eding的回答相反：

如果我在代码中忘记声明myVar，那么我会收到myVar未定义的错误信息。

让我们来看一个真实的例子：

我有一个变量名，但我不确定它是否已经被声明过。

然后@Anurag的回答将会有所帮助：

var myVariableToCheck = 'myVar';
if (window[myVariableToCheck] === undefined)
    console.log("Not declared or declared, but undefined.");

// Or you can check it directly 
if (window['myVar'] === undefined) 
    console.log("Not declared or declared, but 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 

我将它用作函数参数，并在函数执行时排除它，这样我就可以得到“真正”的未定义。虽然这确实需要你将代码放在一个函数内。我是在阅读jQuery源代码时发现了这个方法。

undefined = 2;

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