问题描述
我想检查变量是否已定义。例如，以下内容抛出未定义的错误
I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error
alert( x );
如何捕获此错误？
推荐答案
在JavaScript中， null 是一个对象。对于不存在的事物还有另一个值， undefined 。 DOM几乎在所有无法在文档中找到某些结构的情况下返回 null ，但在JavaScript本身中 undefined 是使用的值。
In JavaScript, null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.
第二，不，没有直接的等价物。如果你真的想特意检查 null ，请执行：
Second, no, there is not a direct equivalent. If you really want to check for specifically for null, do:
if (yourvar === null) // Does not execute if yourvar is `undefined`
如果你想检查一个变量是否存在，只能用尝试 / catch ，因为 typeof 会将未声明的变量和使用 undefined 的值声明的变量视为等效。
If you want to check if a variable exists, that can only be done with try/catch, since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.
但是，要检查变量是否声明且不是未定义：
But, to check if a variable is declared and is not undefined:
if (typeof yourvar !== 'undefined') // Any scope
如果您知道该变量存在，并想检查其中是否存有任何值：
If you know the variable exists, and want to check whether there's any value stored in it:
if (yourvar !== undefined)
如果你想知道一个成员是否独立但不是关心它的价值是什么：
If you want to know if a member exists independent but don't care what its value is:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
如果你想知道变量是否是：
If you want to to know whether a variable is truthy:
if (yourvar)
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