blocks|key|3227900|text|在浏览器控制台中执行它的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3227901|会显示SyntaxError:+Unexpected+**。|blockquote|3227902|3227903|3227904|因为那是spec。这样设计的目的是为了避免混淆它是1的否定的平方(即(-1)+**+2)，还是1的平方的否定(即-(1+**+2))。这种设计是运算符优先的extensive+discussion的结果，并检查了在其他语言中如何处理这一点，最后决定通过将其作为语法错误来避免意外行为。|offset|length|style|CODE|3227905|entityMap|0|LINK|mutability|MUTABLE|url|https://www.ecma-international.org/ecma-262/7.0/#sec-exp-operator|1|https://esdiscuss.org/topic/exponentiation-operator-precedence^0|0|0|0|0|Y|9|1K|9|4|4|0|26|K|1|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$]]|$1|E|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]|$I|13|J|14|K|L]]|9|@$I|15|J|16|1|17]|$I|18|J|19|1|1A]]|A|$]]|$1|M|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]|U|$5|P|Q|R|A|$S|V]]]]

<blockquote>
 Executing it in the browser console says SyntaxError: Unexpected token **.
</blockquote>

Because that's the <a href="https://www.ecma-international.org/ecma-262/7.0/#sec-exp-operator" rel="noreferrer">spec</a>. Designed that way to avoid confusion about whether it's the square of the negation of one (i.e. <code>(-1) ** 2</code>), or the negation of the square of one (i.e. <code>-(1 ** 2)</code>). This design was the result of <a href="https://esdiscuss.org/topic/exponentiation-operator-precedence" rel="noreferrer">extensive discussion</a> of operator precedence, and examination of how this is handled in other languages, and finally the decision was made to avoid unexpected behavior by making this a syntax error.

blocks|key|3227917|text|从documentation+on+MDN|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3227918|在JavaScript中，不可能编写模棱两可的求幂表达式，即不能将一元运算符(%2B/-/~/!/delete/void/typeof)紧跟在基数之前。|style|CODE|3227919|3227920|在同一文本中也解释了原因：|3227921|3227922|在大多数语言中(通常是%5E或**)，幂运算符定义为比一元运算符(如一元%2B和一元-+)具有更高的优先级，但也有少数例外。例如，在Bash中，**操作符被定义为具有比一元操作符更低的优先级。|blockquote|3227923|3227924|3227925|因此，为了避免混淆，决定代码必须消除歧义，并显式地放入括号：|3227926|(-1)**2|code-block|syntax|javascript|3227927|或者：|3227928|-(1**2)+|3227929|注意，二进制-不是这样处理的--优先级较低--因此最后一个表达式的结果与这个有效表达式的结果相同：|3227930|0-1**2|3227931|其他编程语言中的指数优先|3227932|正如在上面的引述中所肯定的那样，大多数具有中缀求幂运算符的编程语言，给予该运算符的优先级高于一元减号。|3227933|下面是其他一些给予一元减号运算符更高优先级的编程语言示例：|3227934|3227935|bc|unordered-list-item|3227936|VBScript|3227937|AppleScript|3227938|COBOL|3227939|Rexx|3227940|Orc|3227941|3227942|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Exponentiation_(**))|1|https://www.gnu.org/software/bc/manual/html_mono/bc.html#SEC13|2|https://msdn.microsoft.com/fr-fr/library/6s7zy3d1(v=vs.84).aspx|3|https://developer.apple.com/library/content/documentation/AppleScript/Conceptual/AppleScriptLangGuide/reference/ASLR_operators.html#//apple_ref/doc/uid/TP40000983-CH5g-SW3|4|https://www.ibm.com/support/knowledgecenter/SS6SG3_4.2.0/com.ibm.entcobol.doc_4.2/PGandLR/tasks/tpari35.htm|5|https://www.ibm.com/support/knowledgecenter/en/SS9UMF_12.1.0/ref/ref/tpc/dsq_rexx_operator.html|6|https://orc.csres.utexas.edu/documentation/html/refmanual/ref.syntax.precedence.html^0|1|K|0|0|13|1|15|1|17|1|19|1|1B|6|1I|4|1N|6|0|0|0|0|B|1|D|2|Y|1|12|1|1W|2|0|0|0|0|0|0|0|6|1|0|0|0|0|0|0|0|2|1|0|0|8|2|0|0|B|3|0|0|5|4|0|0|4|5|0|0|3|6|0|0^^$0|@$1|2|3|4|5|6|7|2A|8|@]|9|@$A|2B|B|2C|1|2D]]|C|$]]|$1|D|3|E|5|6|7|2E|8|@$A|2F|B|2G|F|G]|$A|2H|B|2I|F|G]|$A|2J|B|2K|F|G]|$A|2L|B|2M|F|G]|$A|2N|B|2O|F|G]|$A|2P|B|2Q|F|G]|$A|2R|B|2S|F|G]]|9|@]|C|$]]|$1|H|3|-4|5|6|7|2T|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|2U|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|2V|8|@]|9|@]|C|$]]|$1|L|3|M|5|N|7|2W|8|@$A|2X|B|2Y|F|G]|$A|2Z|B|30|F|G]|$A|31|B|32|F|G]|$A|33|B|34|F|G]|$A|35|B|36|F|G]]|9|@]|C|$]]|$1|O|3|-4|5|6|7|37|8|@]|9|@]|C|$]]|$1|P|3|-4|5|6|7|38|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|39|8|@]|9|@]|C|$]]|$1|S|3|T|5|U|7|3A|8|@]|9|@]|C|$V|W]]|$1|X|3|Y|5|6|7|3B|8|@]|9|@]|C|$]]|$1|Z|3|10|5|U|7|3C|8|@]|9|@]|C|$V|W]]|$1|11|3|12|5|6|7|3D|8|@$A|3E|B|3F|F|G]]|9|@]|C|$]]|$1|13|3|14|5|U|7|3G|8|@]|9|@]|C|$V|W]]|$1|15|3|16|5|6|7|3H|8|@]|9|@]|C|$]]|$1|17|3|18|5|6|7|3I|8|@]|9|@]|C|$]]|$1|19|3|1A|5|6|7|3J|8|@]|9|@]|C|$]]|$1|1B|3|-4|5|6|7|3K|8|@]|9|@]|C|$]]|$1|1C|3|1D|5|1E|7|3L|8|@]|9|@$A|3M|B|3N|1|3O]]|C|$]]|$1|1F|3|1G|5|1E|7|3P|8|@]|9|@$A|3Q|B|3R|1|3S]]|C|$]]|$1|1H|3|1I|5|1E|7|3T|8|@]|9|@$A|3U|B|3V|1|3W]]|C|$]]|$1|1J|3|1K|5|1E|7|3X|8|@]|9|@$A|3Y|B|3Z|1|40]]|C|$]]|$1|1L|3|1M|5|1E|7|41|8|@]|9|@$A|42|B|43|1|44]]|C|$]]|$1|1N|3|1O|5|1E|7|45|8|@]|9|@$A|46|B|47|1|48]]|C|$]]|$1|1P|3|-4|5|6|7|49|8|@]|9|@]|C|$]]|$1|1Q|3|-4|5|6|7|4A|8|@]|9|@]|C|$]]]|1R|$1S|$5|1T|1U|1V|C|$1W|1X]]|1Y|$5|1T|1U|1V|C|$1W|1Z]]|20|$5|1T|1U|1V|C|$1W|21]]|22|$5|1T|1U|1V|C|$1W|23]]|24|$5|1T|1U|1V|C|$1W|25]]|26|$5|1T|1U|1V|C|$1W|27]]|28|$5|1T|1U|1V|C|$1W|29]]]]

From the <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Exponentiation_(**)" rel="nofollow noreferrer">documentation on MDN</a>:

<blockquote>
 In JavaScript, it is impossible to write an ambiguous exponentiation expression, i.e. you cannot put a unary operator (<code>+</code>/<code>-</code>/<code>~</code>/<code>!</code>/<code>delete</code>/<code>void</code>/<code>typeof</code>) immediately before the base number.
</blockquote>

The reason is also explained in that same text:

<blockquote>
 In most languages like PHP and Python and others that have an exponentiation operator (typically <code>^</code> or <code>**</code>), the exponentiation operator is defined to have a higher precedence than unary operators such as unary <code>+</code> and unary <code>-</code>, but there are a few exceptions. For example, in Bash the <code>**</code> operator is defined to have a lower precedence than unary operators. 
</blockquote>

So to avoid confusion it was decided that the code must remove the ambiguity and explicitly put the parentheses:

<pre><code>(-1)**2
</code></pre>

or:

<pre><code>-(1**2) 
</code></pre>

As a side note, the binary <code>-</code> is not treated that way -- having lower precedence -- and so the last expression has the same result as this valid expression:

<pre><code>0-1**2
</code></pre>

<h3>Exponentiation Precedence in Other Programming Languages</h3>

As already affirmed in above quote, most programming languages that have an infix exponentiation operator, give a higher precedence to that operator than to the unary minus. 

Here are some other examples of programming languages that give a higher precedence to the unary minus operator:

<ul>
<li><a href="https://www.gnu.org/software/bc/manual/html_mono/bc.html#SEC13" rel="nofollow noreferrer">bc</a></li>
<li><a href="https://msdn.microsoft.com/fr-fr/library/6s7zy3d1(v=vs.84).aspx" rel="nofollow noreferrer">VBScript</a></li>
<li><a href="https://developer.apple.com/library/content/documentation/AppleScript/Conceptual/AppleScriptLangGuide/reference/ASLR_operators.html#//apple_ref/doc/uid/TP40000983-CH5g-SW3" rel="nofollow noreferrer">AppleScript</a></li>
<li><a href="https://www.ibm.com/support/knowledgecenter/SS6SG3_4.2.0/com.ibm.entcobol.doc_4.2/PGandLR/tasks/tpari35.htm" rel="nofollow noreferrer">COBOL</a></li>
<li><a href="https://www.ibm.com/support/knowledgecenter/en/SS9UMF_12.1.0/ref/ref/tpc/dsq_rexx_operator.html" rel="nofollow noreferrer">Rexx</a></li>
<li><a href="https://orc.csres.utexas.edu/documentation/html/refmanual/ref.syntax.precedence.html" rel="nofollow noreferrer">Orc</a></li>
</ul>

Executing it in the browser console it says <code>SyntaxError: Unexpected token **</code>.
Trying it in node:

<pre><code>&gt; -1**2
...
...
...
...^C
</code></pre>

I thought this is an arithmetic expression where <code>**</code> is the power operator. There is no such issue with other operators.

Strangely, typing <code>*/</code> on the second line triggers the execution:

<pre><code>&gt; -1**2
... */
-1**2
 ^^
SyntaxError: Unexpected token **
</code></pre>

What is happening here?

Why is -1**2 a syntax error in JavaScript?

在浏览器控制台中执行时，它显示为SyntaxError: Unexpected token **。在节点中尝试：> -1**2............^C我以为这是一个算术表达式，其中**是幂运算符。其他操作员没有这样的问题。奇怪的是，在第二行输入*/会触发执行：> -1**2... */-1**2  ^^Syntax...

问为什么-1**2是JavaScript中的语法错误？
EN

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问为什么-1**2是JavaScript中的语法错误？
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