javascript - JavaScript null检查(JavaScript null check)

Question

I've come across the following code:

(我遇到过以下代码：)

function test(data) {
    if (data != null && data !== undefined) {
        // some code here
    }
}

I'm somewhat new to JavaScript, but, from other questions I've been reading here, I'm under the impression that this code does not make much sense.

(我对JavaScript很新，但是，从我在这里读过的其他问题来看，我的印象是这段代码没有多大意义。)

---

In particular, this answer states that(特别是， 这个答案说明了这一点)

You'll get an error if you access an undefined variable in any context other than typeof .(如果在typeof以外的任何上下文中访问未定义的变量，则会出现错误。)

Update: The (quote of the) answer above may be misleading.

(更新：上述答案（引用）可能会产生误导。)

It should say ?an undeclared variable? , instead of ?an undefined variable? .

(应该说“一个未声明的变量” ，而不是“一个未定义的变量” 。)

As I found out, in the answers by Ryan ? , maerics , and nwellnhof , even when no arguments are provided to a function, its variables for the arguments are always declared.

(正如我在Ryan? ， maerics和nwellnhof的回答中所发现的那样，即使没有为函数提供参数，也始终声明其参数的变量。)

This fact also proves wrong the first item in the list below.

(这个事实也证明了下面列表中的第一项是错误的。)

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From my understanding, the following scenarios may be experienced:

(根据我的理解，可能会遇到以下情况：)

• The function was called with no arguments, thus making data an undefined variable, and raising an error on data != null .(调用该函数时没有参数，从而使data成为未定义的变量，并在data != null上引发错误data != null 。)

• The function was called specifically with null (or undefined ), as its argument, in which case data != null already protects the inner code, rendering && data !== undefined useless.

  (该函数专门用null （或undefined ）作为参数调用，在这种情况下， data != null已经保护内部代码，渲染&& data !== undefined无用。)

• The function was called with a non-null argument, in which case it will trivially pass both data != null and data !== undefined .

  (该函数使用非null参数调用，在这种情况下，它将平凡传递data != null 和 data !== undefined 。)

Q: Is my understanding correct?

(问：我的理解是否正确？)

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I've tried the following, in Firefox's console:

(我在Firefox的控制台中尝试了以下内容：)

--
[15:31:31.057] false != null
[15:31:31.061] true
--
[15:31:37.985] false !== undefined
[15:31:37.989] true
--
[15:32:59.934] null != null
[15:32:59.937] false
--
[15:33:05.221] undefined != null
[15:33:05.225] false
--
[15:35:12.231] "" != null
[15:35:12.235] true
--
[15:35:19.214] "" !== undefined
[15:35:19.218] true

I can't figure out a case where the data !== undefined after data != null might be of any use.

(我无法弄清楚data !== undefined data != null 后的情况 data != null可能有用。)

  ask by afsantos translate from so

Answer 1

An “undefined variable” is different from the value undefined .

(“未定义变量”与undefined值不同。)

An undefined variable:

(未定义的变量：)

var a;
alert(b); // ReferenceError: b is not defined

A variable with the value undefined :

(值为undefined变量：)

var a;
alert(a); // Alerts “undefined”

When a function takes an argument, that argument is always declared even if its value is undefined , and so there won't be any error.

(当一个函数接受一个参数时，即使它的值undefined ，该参数也总是被声明，因此不会有任何错误。)

You are right about != null followed by !== undefined being useless, though.

(你是对的!= null后跟!== undefined无用。)