问题

我正在尝试编写一个 if 语句，如果输入已定义，则打印“input is defined”，否则打印 undefined。

function isItUndefined(param)
{
console.log(typeof(param))
if (typeof(param) !== undefined)
{
return 'Input is defined'
}
return undefined
}
console.log(isItUndefined(5))
console.log(isItUndefined(null))
console.log(isItUndefined(undefined))

上面的代码然而给出了以下输出，因为即使条件为假时，默认语句也不会执行：

number
Input is defined
object
Input is defined
undefined
Input is defined

<details>
<summary>英文:</summary>

I am trying to write an if statement that prints &quot;input is defined&quot; if the input is defined, otherwise, undefined is printed.

function isItUndefined(param)
{
console.log(typeof(param))
if (typeof(param) !== undefined)
{
return 'Input is defined'
}
return undefined
}
console.log(isItUndefined(5))
console.log(isItUndefined(null))
console.log(isItUndefined(undefined))

The above code however gives the following output, as the default statement does not execute even when the condition is false:

number
Input is defined
object
Input is defined
undefined
Input is defined

</details>


# 答案1
**得分**: 2

```javascript
`typeof` 返回一个 `string`，所以你的代码应该是：

```javascript
if (typeof(param) !== 'undefined') {
  // 打印
}

MDN 实际上有一个专门的页面介绍如何检测 undefined 值以及你可能在每种方法中遇到的问题。例如，在这里使用 typeof 可能会导致一个相对容易通过单元测试捕捉的 bug：如果你在 param 中写错了一个字母，这个 if 语句几乎总是会为 true。

<details>
<summary>英文:</summary>

`typeof` returns a `string`, so your code should be:

```javascript
if (typeof(param) !== &#39;undefined&#39;) {
  // Print
}

MDN actually has a dedicated page on how to detect undefined values and what kind of hiccups you might run into with each method. For example, using typeof here might actually cause a bug that's relatively easy to catch with a unit test: if you made a typo in param, this if statement is going to be true almost always.

答案2

得分: 1

The typeof operator returns a string, so either compare the input directly with the value undefined, or compare typeof param with the string 'undefined'.

if (typeof param !== 'undefined') 
// or
if (param !== undefined)

if (typeof param !== 'undefined') 
// or
if (param !== undefined)

答案3

得分: 0

你不需要使用 typeof 就能做到这一点。

function isItUndefined(param) {
    return param === undefined ? "undefined" : "输入已定义"; 
    // 如果目标是打印返回值，你可能更喜欢在两种情况下都返回一个字符串，以保持一致的返回类型。
}

console.log(isItUndefined(5))
console.log(isItUndefined(null))
console.log(isItUndefined(undefined))
console.log(isItUndefined())

function isItUndefined(param) {
   return param === undefined ? "undefined" : "Input is defined"; 

console.log(isItUndefined(5))
console.log(isItUndefined(null))
console.log(isItUndefined(undefined))
console.log(isItUndefined())