如何检查“未定义”在JavaScript？ [英] How to check for "undefined" in JavaScript?

问题描述

测试JavaScript中是否未定义变量的最合适方法是什么？我见过几种可能的方法：

What is the most appropriate way to test if a variable is undefined in JavaScript? I've seen several possible ways:

if (window.myVariable)

或

if (typeof(myVariable) != "undefined")

或

if (myVariable) //This throws an error if undefined. Should this be in Try/Catch?

推荐答案

如果您有兴趣了解变量是否有变量已声明无论其值如何，然后在运算符中使用是最安全的方法。考虑这个例子。

If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example.

// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"

但这可能不是某些情况下的预期结果，因为变量或属性已声明但尚未初始化。使用运算符中的进行更强大的检查。

But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in operator for a more robust check.

"theFu" in window; // true
"theFoo" in window; // false

如果您有兴趣知道变量是否尚未声明或具有该值 undefined ，然后使用 typeof 运算符。

If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator.

if (typeof myVar !== 'undefined')

typeof 运算符保证返回一个字符串。直接比较 undefined 很麻烦，因为 undefined 可以被覆盖。

The typeof operator is guaranteed to return a string. Direct comparisons against undefined are troublesome as undefined can be overwritten.

window.undefined = "omg";
"omg" == undefined // true

正如@CMS所指出的，这有在ECMAScript第5版中修补过， undefined 是不可写的。

As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable.

if （window.myVar）还会包含这些假值，因此它不是很强大：

if (window.myVar) will also include these falsy values, so it's not very robust:

false
0
""
NaN
null
undefined

感谢@CMS指出你的第三种情况 - if（myVariable）在两种情况下也会抛出错误。第一个是当尚未定义抛出 ReferenceError 的变量时。

Thanks to @CMS for pointing out that your third case - if (myVariable) can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError.

// abc was never declared.
if (abc) {
    // ReferenceError: abc is not defined
} 

另一种情况是定义了变量，但是有一个getter函数，在调用时会抛出一个错误。例如，

The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,

// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", { 
    get: function() { throw new Error("W00t?"); }, 
    set: undefined 
});
if (myVariable) {
    // Error: W00t?
}

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