如何检查“未定义"?在JavaScript中? [英] How can I check for "undefined" in JavaScript?

问题描述

测试变量是否在JavaScript中未定义的最合适方法是什么?

What is the most appropriate way to test if a variable is undefined in JavaScript?

我已经看到了几种可能的方法:

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

如果您有兴趣找出是否已声明变量而无论其值如何，那么使用in运算符是最安全的方法.考虑以下示例:

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"

但这在某些情况下可能不是预期的结果，因为已声明但未初始化变量或属性.使用in运算符进行更可靠的检查.

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

如果您想知道变量是否尚未声明或值是否为undefined，请使用typeof运算符，该运算符可确保返回字符串:

If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string:

if (typeof myVar !== 'undefined')

与undefined的直接比较比较麻烦，因为undefined可能会被覆盖.

Direct comparisons against undefined are troublesome as undefined can be overwritten.

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

@CMS指出，此问题已在ECMAScript第5版中进行了修补，并且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)还将包含这些伪造的值，因此它不是很可靠:

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

false
0
""
NaN
null
undefined

感谢@CMS指出您的第三种情况-if (myVariable)在两种情况下也会引发错误.第一个是未定义变量时抛出ReferenceError的情况.

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
} 

另一种情况是变量已定义但具有getter函数，该函数在调用时会引发错误.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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