javascript - 如何在JavaScript中检查未定义或null变量？(How to check for an undefined or null variable in JavaScript?)

Question

We are frequently using the following code pattern in our JavaScript code

(我们经常在JavaScript代码中使用以下代码模式)

if (typeof(some_variable) != 'undefined' && some_variable != null)
{
    // Do something with some_variable
}

Is there a less verbose way of checking that has the same effect?

(有没有更冗长的检查方法具有相同的效果？)

According to some forums and literature saying simply the following should have the same effect.

(根据一些论坛和文献的说法，简单地讲，以下应该具有相同的效果。)

if (some_variable)
{
    // Do something with some_variable
}

Unfortunately, Firebug evaluates such a statement as error on runtime when some_variable is undefined, whereas the first one is just fine for it.

(不幸的是，当未定义some_variable时， Firebug some_variable这样的语句评估为运行时错误，而第一个语句就可以了。)

Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways?

(这仅仅是Firebug的一种（有害的）行为，还是这两种方式之间确实存在一些区别？)

  ask by Tomas Vana translate from so

Answer 1

I think the most efficient way to test for "value is null or undefined " is

(我认为测试“值是否为null或undefined ”的最有效方法是)

if ( some_variable == null ){
  // some_variable is either null or undefined
}

So these two lines are equivalent:

(因此，这两行是等效的：)

if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}

Note 1

(注1)

As mentioned in the question, the short variant requires that some_variable has been declared, otherwise a ReferenceError will be thrown.

(正如问题中提到的那样，简短变体要求声明了some_variable ，否则将引发ReferenceError。)

However in many use cases you can assume that this is safe:

(但是，在许多用例中，您可以假定这是安全的：)

check for optional arguments:

(检查可选参数：)

function(foo){
    if( foo == null ) {...}

check for properties on an existing object

(检查现有对象的属性)

if(my_obj.foo == null) {...}

On the other hand typeof can deal with undeclared global variables (simply returns undefined ).

(另一方面， typeof可以处理未声明的全局变量（仅返回undefined ）。)

Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained.

(正如Alsciende解释的那样，出于充分的原因，这些案例应该减少到最少。)

Note 2

(笔记2)

This - even shorter - variant is not equivalent:

(这个-甚至更短-的变体并不等效：)

if ( !some_variable ) {
  // some_variable is either null, undefined, 0, NaN, false, or an empty string
}

so

(所以)

if ( some_variable ) {
  // we don't get here if some_variable is null, undefined, 0, NaN, false, or ""
}

Note 3

(注3)

In general it is recommended to use === instead of == .

(通常，建议使用===代替== 。)

The proposed solution is an exception to this rule.

(提议的解决方案是该规则的例外。)

The JSHint syntax checker even provides the eqnull option for this reason.

(为此， eqnull 语法检查器甚至提供了eqnull选项。)

From the jQuery style guide :

(从jQuery样式指南 ：)

Strict equality checks (===) should be used in favor of ==.

(应该使用严格的相等性检查（===）来支持==。)

The only exception is when checking for undefined and null by way of null.

(唯一的例外是通过null检查undefined和null时。)

 // Check for both undefined and null values, for some important reason. undefOrNull == null;