blocks|key|267527|text|您可以使用eval()|type|unstyled|depth|inlineStyleRanges|entityRanges|data|267528|方法计算表示为字符串的JavaScript代码。|blockquote|267529|将代码更改为：|267530|$('.btn').click(function(){
++++var+selected+=+eval($('#display').val());
++++alert(selected);
});|code-block|syntax|javascript|267531|还请注意：|267532|eval()是一个危险的函数，它以调用者的权限执行传递的代码。|267533|请在MDN上阅读更多信息，以了解涉及eval的风险。|offset|length|style|BOLD|267534|对象/评估|267535|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval^0|0|0|0|0|0|0|I|4|0|0|5|0|0^^$0|@$1|2|3|4|5|6|7|15|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|16|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|17|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|18|8|@]|9|@]|A|$J|K]]|$1|L|3|M|5|6|7|19|8|@]|9|@]|A|$]]|$1|N|3|O|5|D|7|1A|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1B|8|@$R|1C|S|1D|T|U]]|9|@]|A|$]]|$1|V|3|W|5|6|7|1E|8|@]|9|@$R|1F|S|1G|1|1H]]|A|$]]|$1|X|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14]]]]

<h2>You can use eval()</h2>
<blockquote>
The eval() method evaluates JavaScript code represented as a string.
</blockquote>
Change your code to this:
<pre><code>$('.btn').click(function(){
 var selected = eval($('#display').val());
 alert(selected);
});
</code></pre>
Also note:
<blockquote>
eval() is a dangerous function, which executes the code it's passed with the privileges of the caller.
</blockquote>
Read more at MDN to understand the risks involving eval.
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval" rel="nofollow noreferrer">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval</a>

blocks|key|162751|text|更新Fiddle|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|162752|$('.btn').click(function(){
++++var+selected+=+$('#display').val();
++++alert(eval(selected));
});|code-block|syntax|javascript|162753|不要忘记添加适当的错误处理，以避免在存在兼容值的情况下出现错误。|162754|编辑：+eval函数已经知道安全漏洞。最好不要使用它。|style|BOLD|CODE|162755|$('.btn').click(function(){
++++var+selected+=+$('#display').val();
++++alert(evil(selected));
});

function+evil(fn)+{
++++return+new+Function('return+'+%2B+fn)();
}|162756|这是我在另一个线程中找到的另一个解决方案。|162757|entityMap|0|LINK|mutability|MUTABLE|url|https://jsfiddle.net/pt44c31p/3/|1|http://24ways.org/2005/dont-be-eval|2|https://stackoverflow.com/a/18082175/1479535^0|0|8|0|0|0|0|0|3|4|4|E|4|1|0|0|4|5|2|0^^$0|@$1|2|3|4|5|6|7|15|8|@]|9|@$A|16|B|17|1|18]]|C|$]]|$1|D|3|E|5|F|7|19|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1A|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|1B|8|@$A|1C|B|1D|M|N]|$A|1E|B|1F|M|O]]|9|@$A|1G|B|1H|1|1I]]|C|$]]|$1|P|3|Q|5|F|7|1J|8|@]|9|@]|C|$G|H]]|$1|R|3|S|5|6|7|1K|8|@]|9|@$A|1L|B|1M|1|1N]]|C|$]]|$1|T|3|-4|5|6|7|1O|8|@]|9|@]|C|$]]]|U|$V|$5|W|X|Y|C|$Z|10]]|11|$5|W|X|Y|C|$Z|12]]|13|$5|W|X|Y|C|$Z|14]]]]

<a href="https://jsfiddle.net/pt44c31p/3/" rel="nofollow noreferrer">Updated Fiddle</a>

<pre><code>$('.btn').click(function(){
 var selected = $('#display').val();
 alert(eval(selected));
});
</code></pre>

Don't forget to add proper error handling to avoid errors in the situations where there are in compatible values.

Edit: 
The <code>eval</code> function has known <a href="http://24ways.org/2005/dont-be-eval" rel="nofollow noreferrer">security vulnerabilities</a>. It is always advisable not to use it.

<pre><code>$('.btn').click(function(){
 var selected = $('#display').val();
 alert(evil(selected));
});

function evil(fn) {
 return new Function('return ' + fn)();
}
</code></pre>

This is another solution I found in <a href="https://stackoverflow.com/a/18082175/1479535">another thread</a>.

blocks|key|1322387|text|您之所以使5%2B5处于警报状态是因为您的“表达式”未被执行。会执行参数的位置(Args)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1322388|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|$]]

Reason why you are getting 5+5 in alert is because your 'expression' is not executed. Where eval(args) will execute the arguments.

blocks|key|1322401|text|只要您像下面这样清理输入字符串，就可以在本例中使用eval()：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|1322402|$('.btn').click(function(){
++++//+strip+anything+other+than+digits,+(),+-%2B/*+and+.
++++var+selected+=+$('#display').val().replace(/[%5E-()\d/*%2B.]/g,+'');
++++alert(eval(selected+));
});|code-block|syntax|javascript|1322403|您还可以参考这条线来获得eval()的其他替代方案。我们已经详细讨论了使用eval()的安全风险。|1322404|干杯!|1322405|编辑:更新的小提琴链接这里进行测试。|1322406|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/6479236/calculate-string-value-in-javascript-not-using-eval|1|https://jsfiddle.net/pt44c31p/8/^0|P|6|0|0|C|6|11|6|6|3|0|0|0|B|2|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@$9|19|A|1A|1|1B]]|E|$]]|$1|M|3|N|5|6|7|1C|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1D|8|@]|D|@$9|1E|A|1F|1|1G]]|E|$]]|$1|Q|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]|Y|$5|T|U|V|E|$W|Z]]]]

eval() can be used in this case as long as you sanitize the input string like below:

<pre><code>$('.btn').click(function(){
 // strip anything other than digits, (), -+/* and .
 var selected = $('#display').val().replace(/[^-()\d/*+.]/g, '');
 alert(eval(selected ));
});
</code></pre>

You can also refer to <a href="https://stackoverflow.com/questions/6479236/calculate-string-value-in-javascript-not-using-eval">this thread</a> for other alternatives to eval(). The security risks of using eval() has been discussed <a href="https://stackoverflow.com/questions/86513/why-is-using-the-javascript-eval-function-a-bad-idea">here</a> in great detail.

Cheers!

EDIT: Updated fiddle link <a href="https://jsfiddle.net/pt44c31p/8/" rel="nofollow noreferrer">here</a> for testing.

i have the following Html

<pre><code>&lt;input class="top-display" value="5+5" id="display" disabled="disabled"&gt; &lt;/input&gt;
&lt;button class="btn" &gt; submit &lt;/button&gt;
</code></pre>

And jquery

<pre><code>$('.btn').click(function(){
 var selected = $('#display').val();
 alert(selected);

});
</code></pre>

<a href="https://jsfiddle.net/pt44c31p/2/" rel="nofollow">fiddle Here</a>
my focus for this code is letting jquery to automatically do the math for me. like instead of jquery alerting "5+5" it should alert "10". and if the value = "5-5" is should alert "0"; without adding any if statement.

using jquery to calculate HTML input value automatically

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。

页面排版不美观，阅读体验差。

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我有以下Html<input class="top-display" value="5+5" id="display" disabled="disabled"> </input><button class="btn" > submit </button>和jquery$('.btn').click(function(){ var selected = $('#display').val();

问使用jquery自动计算HTML输入值
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