blocks|key|686044|text|如果if语句不在if语句之外使用，则在if语句中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|686045|entityMap^0|2|2|8|2|J|2|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

<p>Inside the <code>if</code> statement if it is not used outside the <code>if</code> statement.</p>


blocks|key|1005066|text|把它放在if语句中。特别是如果变量赋值成本很高，因为除非满足if语句，否则它永远不会被使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1005067|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

<p>Put it within the if statement. Especially if the variable assignment is costly as it is never being used unless the if statement is satisfied. </p>


blocks|key|1005069|text|在if语句中。根据您的代码，IF语句只需要lastMessage函数，因此您应该将它括起来。否则，您将在每次运行if检查时执行计算，而不管是否需要它。那太浪费了。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1005070|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

<p>Inside the if statement. The lastMessage function is required by the IF statement only, according to your code - so you should enclose it. Otherwise you are performing a computation every time you run your if check, regardless of whether or not you are going to need it. That would be wasteful. </p>


blocks|key|1005108|text|是的，绝对|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1005109|正如许多JavaScript风格指南中所述：|1005110|Airbnb+JavaScript样式指南(){|offset|length|1005111|13.4在需要变量的地方分配变量，但将它们放在合理的位置。为什么？let和const是块作用域，而不是函数作用域。|blockquote|1005112|书写一致的、惯用的JavaScript原则|1005113|//+2.B.1.4
//+const+and+let,+from+ECMAScript+6,+should+likewise+be+at+the+top+of+their+scope+(block).

//+Bad
function+foo()+{
++let+foo,
++++bar;
++if+(+condition+)+{
++++bar+=+"";
++++//+statements
++}
}
//+Good
function+foo()+{
++let+foo;
++if+(+condition+)+{
++++let+bar+=+"";
++++//+statements
++}
}|code-block|syntax|javascript|1005114|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/airbnb/javascript#13.4|1|https://github.com/rwaldron/idiomatic.js^0|0|0|0|O|0|0|0|0|L|1|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|13|8|@]|9|@$F|14|G|15|1|16]]|A|$]]|$1|H|3|I|5|J|7|17|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|18|8|@]|9|@$F|19|G|1A|1|1B]]|A|$]]|$1|M|3|N|5|O|7|1C|8|@]|9|@]|A|$P|Q]]|$1|R|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|S|$T|$5|U|V|W|A|$X|Y]]|Z|$5|U|V|W|A|$X|10]]]]

<h2>Yes, Definitely</h2>
<p>As stated in many JavaScript style guides:</p>
<hr />
<h3><a href="https://github.com/airbnb/javascript#13.4" rel="nofollow noreferrer">Airbnb JavaScript Style Guide() {</a></h3>
<blockquote>13.4 Assign variables where you need them, but place them in a reasonable place.
    <blockquote>Why? let and const are block scoped and not function scoped.</blockquote>
</blockquote>
<hr />
<h3><a href="https://github.com/rwaldron/idiomatic.js" rel="nofollow noreferrer">Principles of Writing Consistent, Idiomatic JavaScript</a></h3>
<pre><code>// 2.B.1.4
// const and let, from ECMAScript 6, should likewise be at the top of their scope (block).

// Bad
function foo() {
  let foo,
    bar;
  if ( condition ) {
    bar = &quot;&quot;;
    // statements
  }
}
// Good
function foo() {
  let foo;
  if ( condition ) {
    let bar = &quot;&quot;;
    // statements
  }
}
</code></pre>


<p>I want to know, according to good practices, what is the most convenient way to put variables in this case: </p>

<p>If you have a function, where its content is going to be executed only if the conditional statement is true, where would you put the variables?</p>

<pre><code>  addMessage (text) {
    let lastMessage = React.findDOMNode(this.refs.messages);
    if (text.length) {
      ChatActions.addMessage(text);
      lastMessage.scrollTop = lastMessage.scrollHeight;
    }
  }
</code></pre>

<p>Or like this:</p>

<pre><code>  addMessage (text) {
    if (text.length) {
      let lastMessage = React.findDOMNode(this.refs.messages);
      ChatActions.addMessage(text);
      lastMessage.scrollTop = lastMessage.scrollHeight;
    }
  }
</code></pre>


Should I put variables inside an if conditional?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

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我想知道，根据良好的实践，在这种情况下放置变量的最方便的方法是什么：如果您有一个函数，只有在条件语句为真的情况下，它的内容才会被执行，那么您将把变量放在哪里？  addMessage (text) {    let lastMessage = React.findDOMNode(this.refs.messages);...

问我是否应该将变量放入一个if条件中？
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问我是否应该将变量放入一个if条件中？EN