如何在 JavaScript 中检查未定义的变量 [英] How to check a not-defined variable in JavaScript

问题描述

我想检查变量是否已定义.例如下面抛出一个未定义的错误

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error

alert( x );

我怎样才能发现这个错误?

How can I catch this error?

推荐答案

在 JavaScript 中，null 是一个对象.不存在的事物还有另一个价值，undefined.DOM 在几乎所有无法在文档中找到某些结构的情况下都会返回 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.如果您真的想专门检查 null，请执行以下操作:

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，那只能用 try/catch 来完成，因为 typeof 会处理一个未声明的变量和一个用 undefined 的值声明为等效的变量.

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，检查一个变量是否被声明并且不是undefined:

But, to check if a variable is declared and is not undefined:

if (yourvar !== undefined) // Any scope

以前，必须使用 typeof 运算符来安全地检查 undefined，因为可以像变量一样重新分配 undefined.旧的方式是这样的:

Previously, it was necessary to use the typeof operator to check for undefined safely, because it was possible to reassign undefined just like a variable. The old way looked like this:

if (typeof yourvar !== 'undefined') // Any scope

undefined 可重新赋值的问题已在 2009 年发布的 ECMAScript 5 中得到修复.您现在可以安全地使用 === 和 ！== 在不使用 typeof 的情况下测试 undefined，因为 undefined 已经只读了一段时间.

The issue of undefined being re-assignable was fixed in ECMAScript 5, which was released in 2009. You can now safely use === and !== to test for undefined without using typeof as undefined has been read-only for some time.

如果你想知道一个成员是否独立存在但不关心它的值是什么:

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

如果你想知道一个变量是否真实:

If you want to to know whether a variable is truthy:

if (yourvar)

来源

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