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GraveAccent 翻译中 Conditional Rendering in React using Ternaries and Logical AND
============================================================
![](https://cdn-images-1.medium.com/max/2000/1*eASRJrCIVgsy5VbNMAzD9w.jpeg)
Photo by [Brendan Church][1] on [Unsplash][2]
There are several ways that your React component can decide what to render. You can use the traditional `if` statement or the `switch` statement. In this article, we’ll explore a few alternatives. But be warned that some come with their own gotchas, if you’re not careful.
### Ternary vs if/else
Let’s say we have a component that is passed a `name` prop. If the string is non-empty, we display a greeting. Otherwise we tell the user they need to sign in.
Here’s a Stateless Function Component (SFC) that does just that.
```
const MyComponent = ({ name }) => {
if (name) {
return (
<div className="hello">
Hello {name}
</div>
);
}
return (
<div className="hello">
Please sign in
</div>
);
};
```
Pretty straightforward. But we can do better. Here’s the same component written using a conditional ternary operator.
```
const MyComponent = ({ name }) => (
<div className="hello">
{name ? `Hello ${name}` : 'Please sign in'}
</div>
);
```
Notice how concise this code is compared to the example above.
A few things to note. Because we are using the single statement form of the arrow function, the `return` statement is implied. Also, using a ternary allowed us to DRY up the duplicate `<div className="hello">` markup. 🎉
### Ternary vs Logical AND
As you can see, ternaries are wonderful for `if/else` conditions. But what about simple `if` conditions?
Let’s look at another example. If `isPro` (a boolean) is `true`, we are to display a trophy emoji. We are also to render the number of stars (if not zero). We could go about it like this.
```
const MyComponent = ({ name, isPro, stars}) => (
<div className="hello">
<div>
Hello {name}
{isPro ? '🏆' : null}
</div>
{stars ? (
<div>
Stars:{'⭐️'.repeat(stars)}
</div>
) : null}
</div>
);
```
But notice the “else” conditions return `null`. This is becasue a ternary expects an else condition.
For simple `if` conditions, we could use something a little more fitting: the logical AND operator. Here’s the same code written using a logical AND.
```
const MyComponent = ({ name, isPro, stars}) => (
<div className="hello">
<div>
Hello {name}
{isPro && '🏆'}
</div>
{stars && (
<div>
Stars:{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
Not too different, but notice how we eliminated the `: null` (i.e. else condition) at the end of each ternary. Everything should render just like it did before.
Hey! What gives with John? There is a `0` when nothing should be rendered. That’s the gotcha that I was referring to above. Here’s why.
[According to MDN][3], a Logical AND (i.e. `&&`):
> `expr1 && expr2`
> Returns `expr1` if it can be converted to `false`; otherwise, returns `expr2`. Thus, when used with Boolean values, `&&` returns `true` if both operands are true; otherwise, returns `false`.
OK, before you start pulling your hair out, let me break it down for you.
In our case, `expr1` is the variable `stars`, which has a value of `0`. Because zero is falsey, `0` is returned and rendered. See, that wasn’t too bad.
I would write this simply.
> If `expr1` is falsey, returns `expr1`, else returns `expr2`.
So, when using a logical AND with non-boolean values, we must make the falsey value return something that React won’t render. Say, like a value of `false`.
There are a few ways that we can accomplish this. Let’s try this instead.
```
{!!stars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
```
Notice the double bang operator (i.e. `!!`) in front of `stars`. (Well, actually there is no “double bang operator”. We’re just using the bang operator twice.)
The first bang operator will coerce the value of `stars` into a boolean and then perform a NOT operation. If `stars` is `0`, then `!stars` will produce `true`.
Then we perform a second NOT operation, so if `stars` is 0, `!!stars` would produce `false`. Exactly what we want.
If you’re not a fan of `!!`, you can also force a boolean like this (which I find a little wordy).
```
{Boolean(stars) && (
```
Or simply give a comparator that results in a boolean value (which some might say is even more semantic).
```
{stars > 0 && (
```
#### A word on strings
Empty string values suffer the same issue as numbers. But because a rendered empty string is invisible, it’s not a problem that you will likely have to deal with, or will even notice. However, if you are a perfectionist and don’t want an empty string on your DOM, you should take similar precautions as we did for numbers above.
### Another solution
A possible solution, and one that scales to other variables in the future, would be to create a separate `shouldRenderStars` variable. Then you are dealing with boolean values in your logical AND.
```
const shouldRenderStars = stars > 0;
```
```
return (
<div>
{shouldRenderStars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
Then, if in the future, the business rule is that you also need to be logged in, own a dog, and drink light beer, you could change how `shouldRenderStars` is computed, and what is returned would remain unchanged. You could also place this logic elsewhere where it’s testable and keep the rendering explicit.
```
const shouldRenderStars =
stars > 0 && loggedIn && pet === 'dog' && beerPref === 'light`;
```
```
return (
<div>
{shouldRenderStars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
### Conclusion
I’m of the opinion that you should make best use of the language. And for JavaScript, this means using conditional ternary operators for `if/else`conditions and logical AND operators for simple `if` conditions.
While we could just retreat back to our safe comfy place where we use the ternary operator everywhere, you now possess the knowledge and power to go forth AND prosper.
--------------------------------------------------------------------------------
作者简介：
Managing Editor at the American Express Engineering Blog http://aexp.io and Director of Engineering @AmericanExpress. MyViews !== ThoseOfMyEmployer.
----------------
via: https://medium.freecodecamp.org/conditional-rendering-in-react-using-ternaries-and-logical-and-7807f53b6935
作者：[Donavon West][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://medium.freecodecamp.org/@donavon
[1]:https://unsplash.com/photos/pKeF6Tt3c08?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[2]:https://unsplash.com/search/photos/road-sign?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[3]:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators

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在 React 条件渲染中使用三元表达式和 “&&”
============================================================
![](https://cdn-images-1.medium.com/max/2000/1*eASRJrCIVgsy5VbNMAzD9w.jpeg)
Photo by [Brendan Church][1] on [Unsplash][2]
React 组件可以通过多种方式决定渲染内容。你可以使用传统的 if 语句或 switch 语句。在本文中，我们将探讨一些替代方案。但要注意，如果你不小心，有些方案会带来自己的陷阱。
### 三元表达式 vs if/else
假设我们有一个组件被传进来一个 `name` prop。 如果这个字符串非空，我们会显示一个问候语。否则，我们会告诉用户他们需要登录。
这是一个只实现了如上功能的无状态函数式组件。
```
const MyComponent = ({ name }) => {
if (name) {
return (
<div className="hello">
Hello {name}
</div>
);
}
return (
<div className="hello">
Please sign in
</div>
);
};
```
这个很简单但是我们可以做得更好。这是使用三元运算符编写的相同组件。
```
const MyComponent = ({ name }) => (
<div className="hello">
{name ? `Hello ${name}` : 'Please sign in'}
</div>
);
```
请注意这段代码与上面的例子相比是多么简洁。
有几点需要注意。因为我们使用了箭头函数的单语句形式，所以隐含了return语句。另外，使用三元运算符允许我们省略掉重复的 `<div className="hello">` 标记。🎉
### 三元表达式 vs &&
正如您所看到的，三元表达式用于表达 if/else 条件式非常好。但是对于简单的 if 条件式怎么样呢？
让我们看另一个例子。如果 isPro（一个布尔值）为真，我们将显示一个奖杯表情符号。我们也要渲染星星的数量（如果不是0）。我们可以这样写。
```
const MyComponent = ({ name, isPro, stars}) => (
<div className="hello">
<div>
Hello {name}
{isPro ? '🏆' : null}
</div>
{stars ? (
<div>
Stars:{'⭐️'.repeat(stars)}
</div>
) : null}
</div>
);
```
请注意 “else” 条件返回 null 。 这是因为三元表达式要有"否则"条件。
对于简单的 “if” 条件式，我们可以使用更合适的东西：&& 运算符。这是使用 “&&” 编写的相同代码。
```
const MyComponent = ({ name, isPro, stars}) => (
<div className="hello">
<div>
Hello {name}
{isPro && '🏆'}
</div>
{stars && (
<div>
Stars:{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
没有太多区别，但是注意我们消除了每个三元表达式最后面的 `: null` （else 条件式）。一切都应该像以前一样渲染。
嘿！约翰得到了什么？当什么都不应该渲染时，只有一个0。这就是我上面提到的陷阱。这里有解释为什么。
[根据 MDN][3]，一个逻辑运算符“和”（也就是`&&`）：
> `expr1 && expr2`
> 如果 `expr1` 可以被转换成 `false` ，返回 `expr1`；否则返回 `expr2`。 如此，当与布尔值一起使用时，如果两个操作数都是 true，`&&` 返回 `true` ；否则，返回 `false`。
好的，在你开始拔头发之前，让我为你解释它。
在我们这个例子里， `expr1` 是变量 `stars`，它的值是 `0`，因为0是 falsey 的值, `0` 会被返回和渲染。看，这还不算太坏。
我会简单地这么写。
> 如果 `expr1` 是 falsey，返回 `expr1` ，否则返回 `expr2`
所以，当对非布尔值使用 “&&” 时，我们必须让 falsy 的值返回 React 无法渲染的东西，比如说，`false` 这个值。
我们可以通过几种方式实现这一目标。让我们试试吧。
```
{!!stars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
```
注意 `stars` 前的双感叹操作符（ `!!`）（呃，其实没有双感叹操作符。我们只是用了感叹操作符两次）。
第一个感叹操作符会强迫 `stars` 的值变成布尔值并且进行一次“非”操作。如果 `stars` 是 `0` ，那么 `!stars` 会 是 `true`。
然后我们执行第二个`非`操作，所以如果 `stars` 是0，`!!stars` 会是 `false`。正好是我们想要的。
如果你不喜欢 `!!`，那么你也可以强制转换出一个布尔数比如这样（这种方式我觉得有点冗长）。
```
{Boolean(stars) && (
```
或者只是用比较符产生一个布尔值（有些人会说这样甚至更加语义化）。
```
{stars > 0 && (
```
#### 关于字符串
空字符串与数字有一样的毛病。但是因为渲染后的空字符串是不可见的，所以这不是那种你很可能会去处理的难题，甚至可能不会注意到它。然而，如果你是完美主义者并且不希望DOM上有空字符串，你应采取我们上面对数字采取的预防措施。
### 其它解决方案
一种可能的将来可扩展到其他变量的解决方案，是创建一个单独的 `shouldRenderStars` 变量。然后你用“&&”处理布尔值。
```
const shouldRenderStars = stars > 0;
```
```
return (
<div>
{shouldRenderStars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
之后，在将来，如果业务规则要求你还需要已登录，拥有一条狗以及喝淡啤酒，你可以改变 `shouldRenderStars` 的得出方式，而返回的内容保持不变。你还可以把这个逻辑放在其它可测试的地方，并且保持渲染明晰。
```
const shouldRenderStars =
stars > 0 && loggedIn && pet === 'dog' && beerPref === 'light`;
```
```
return (
<div>
{shouldRenderStars && (
<div>
{'⭐️'.repeat(stars)}
</div>
)}
</div>
);
```
### 结论
我认为你应该充分利用这种语言。对于 JavaScript，这意味着为 `if/else` 条件式使用三元表达式，以及为 `if` 条件式使用 `&&` 操作符。
我们可以回到每处都使用三元运算符的舒适区，但你现在消化了这些知识和力量，可以继续前进 && 取得成功了。
--------------------------------------------------------------------------------
作者简介：
美国运通工程博客的执行编辑 http://aexp.io 以及 @AmericanExpress 的工程总监。MyViews !== ThoseOfMyEmployer.
----------------
via: https://medium.freecodecamp.org/conditional-rendering-in-react-using-ternaries-and-logical-and-7807f53b6935
作者：[Donavon West][a]
译者：[GraveAccent](https://github.com/GraveAccent)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://medium.freecodecamp.org/@donavon
[1]:https://unsplash.com/photos/pKeF6Tt3c08?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[2]:https://unsplash.com/search/photos/road-sign?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
[3]:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators