blocks|key|41655|text|alert需要一个字符串。如果它没有获得字符串，它将尝试将它接收到的任何对象(而函数是一种对象)转换为字符串。如果对象具有toString方法，则将调用该方法来执行上述转换。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|41656|entityMap^0|0|5|1P|8|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

<code>alert</code> expects a string. If it doesn't get a string, it will attempt to convert whatever object it receives (and a function is a type of object) into one. If the object has a <code>toString</code> method, then that will be called to perform said conversion.

blocks|key|258542|text|需要查看的是这段代码|type|unstyled|depth|inlineStyleRanges|entityRanges|data|258543|function+f(b)+{
++++sum+%2B=+b
++++return+f
++}|code-block|syntax|javascript|258544|此函数返回对自身的引用，因此可以尽可能多次地调用它。关于它的一个重要的事情是它有一个可以被调用的tostring函数，因为tostring是在函数sum()中定义的，所以它可以访问变量sum和它的值current+value+(由f()更改)|offset|length|style|CODE|258545|entityMap^0|0|0|21|5|2K|3|37|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]|$I|V|J|W|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|N|$]]

The thing to look at is this piece of code

<pre><code>function f(b) {
 sum += b
 return f
 }
</code></pre>

This function returns reference to itself so it can be called as many times as possible. An important thing about it is that it has a tostring function that gets called and since tostring is defined inside function <code>sum()</code> it has access to the variable <code>sum</code> and its value current value (that is changed by <code>f()</code> )

blocks|key|41730|text|sum和f函数总是返回f函数，您可以无限次调用它，而不会得到函数以外的结果。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|41731|该值仅由f的重写的toString方法返回(它实际上返回一个数字)：|41732|console.log(+sum(1)(2)+)+//+Function(){}
console.log(+sum(1)(2).toString()+)+//+3|code-block|syntax|javascript|41733|alert函数在将其参数强制转换为字符串时隐式调用toString方法。|41734|entityMap^0|0|3|4|1|B|1|0|4|1|9|8|0|0|0|5|P|8|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|P|$]]

The <code>sum</code> and <code>f</code> functions always return the <code>f</code> function, you can invoke it infinite times without getting a result other than the function.

The value is only returned by the overwritten <code>toString</code> method of <code>f</code> (which actually returns a number):

<pre><code>console.log( sum(1)(2) ) // Function(){}
console.log( sum(1)(2).toString() ) // 3
</code></pre>

The <code>alert</code> function implicitly invokes the <code>toString</code> method when it casts its arguments to strings.

blocks|key|41759|text|第一次调用函数时，第一个值存储在sum中。在此之后，将返回function+f(b)，并在sum中维护临时结果。对于每个连续的调用，您执行函数f+-您执行sum+%2B=+b并再次返回f。如果需要字符串上下文(例如在alert或console.log中)，则调用f.toString，返回结果(sum)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|41760|function+sum(a)+{

++var+sum+=+a

++function+f(b)+{
++++sum+%2B=+b
++++return+f++//<-+from+second+call,+f+is+returned+each+time
++++++++++++++//+++so+you+can+chain+those+calls+indefinitely
++++++++++++++//+++function+sum+basically+got+"overridden"+by+f
++}

++f.toString+=+function()+{+return+sum+}

++return+f+//<-+after+first+call,+f+is+returned
}|code-block|syntax|javascript|41761|解释：|41762|alert(+sum(6)(-1)(-2)(-3)+)++//+0
+++++++++++/\+function+sum+called,+f+returned
++++++++++++++/\+the+returned+function+f+is+called,+f+returns+itself
++++++++++++++++++/\+again
+++++++++++++++++++++/\+and+again
+++++++++++++++++++++++++/\+at+last,+alert()+requires+string+context,
++++++++++++++++++++++++++++so+f.toString+is+getting+invoked+now+instead+of+f|41763|entityMap^0|G|3|T|D|19|3|1Z|1|25|8|2Y|5|34|B|3L|A|41|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|P|$]]

The first time your function is called, the first value is stored in <code>sum</code>. After that <code>function f(b)</code> will be returned, maintaining the provisional result in <code>sum</code>. With each consecutive call you execute function <code>f</code> - you perform <code>sum += b</code> and return f again. If a string context is required (such as in the <code>alert</code>, or a <code>console.log</code>) <code>f.toString</code> is called instead, returning the result (<code>sum</code>).

<pre><code>function sum(a) {

 var sum = a

 function f(b) {
 sum += b
 return f //&lt;- from second call, f is returned each time
 // so you can chain those calls indefinitely
 // function sum basically got "overridden" by f
 }

 f.toString = function() { return sum }

 return f //&lt;- after first call, f is returned
}
</code></pre>

Explanation:

<pre><code>alert( sum(6)(-1)(-2)(-3) ) // 0
 /\ function sum called, f returned
 /\ the returned function f is called, f returns itself
 /\ again
 /\ and again
 /\ at last, alert() requires string context,
 so f.toString is getting invoked now instead of f
</code></pre>

blocks|key|41819|text|它并不是在所有情况下都能像预期的那样工作。问题是.toString期望返回一个字符串，所以在所提供的实现中，字符串方法将不起作用，例如，sum(2)(3).split()将导致错误。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|41820|虽然我们可能会假设sum()结果总是一个数字，但在某些情况下可能不是真的，并且可能很难调试，例如，当我在jsbin.com上测试最初使用.toString编写的代码时，我注意到了这个问题(它在内部对console.log参数执行split，覆盖它)。|41821|相反，.toString应该看起来像return+String(result);。好消息是，.toString+(当没有.valueOf或现代的Symbol.toPrimitive时)将处理原语转换，因此期望数字的代码也可以工作。这里可能出现的问题可能是由此导致的“双倍”转换。|41822|更好的解决方案可能是使用一对.toString和.valueOf，或者如果您只针对现代浏览器，则只使用一个Symbol.toPrimitive。|41823|使用Symbol.toPrimitive的示例|41824|function+sum(a)+{
++let+result+=+a;

++function+f(b)+{
++++result+%2B=+b;

++++return+f;
++}

++f[Symbol.toPrimitive]+=+hint+=>+hint+===+'string'+?+String(result)+:+result;

++return+f;
}|code-block|syntax|javascript|41825|使用.toString和.valueOf对的示例。|41826|function+sum(a)+{
++var+result+=+a;

++function+f(b)+{
++++result+%2B=+b;

++++return+f;
++}

++//+avoiding+double+conversion+which+will+happen+in+case+of+.toString
++f.valueOf+=+function()+{+return+result;+};
++f.toString+=+function()+{+return+String(result);+};

++return+f;
}|41827|entityMap^0|O|9|1W|H|0|9|5|1W|9|36|5|0|3|9|I|M|1A|9|1O|8|20|I|0|E|9|O|8|1H|I|0|2|I|0|0|2|9|C|8|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1L|8|@$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|C]]|D|@]|E|$]]|$1|L|3|M|5|6|7|1S|8|@$9|1T|A|1U|B|C]]|D|@]|E|$]]|$1|N|3|O|5|P|7|1V|8|@]|D|@]|E|$Q|R]]|$1|S|3|T|5|6|7|1W|8|@$9|1X|A|1Y|B|C]|$9|1Z|A|20|B|C]]|D|@]|E|$]]|$1|U|3|V|5|P|7|21|8|@]|D|@]|E|$Q|R]]|$1|W|3|-4|5|6|7|22|8|@]|D|@]|E|$]]]|X|$]]

It doesn't work as intended in all of the cases... The problem is that <code>.toString</code> is expected to return a string, so string methods in provided implementation, would not work, e. g. <code>sum(2)(3).split()</code> will cause an error.

Although we might assume <code>sum()</code> result will always be expected to be a number, it might not be true in some cases and might be hard to debug, e. g. I noticed the issue when I was testing code initially written with <code>.toString</code> only on jsbin.com (it does <code>split</code> on console.log argument internally, overriding it).

Instead, <code>.toString</code> should look like <code>return String(result);</code>. Good thing that <code>.toString</code> (when there's no <code>.valueOf</code> or modern <code>Symbol.toPrimitive</code>) will handle primitives conversion, so code expecting a Number will work as well. The possible issue here might be "double" conversion caused by this.

Better solution might be to use either pair of <code>.toString</code> and <code>.valueOf</code> or just a single <code>Symbol.toPrimitive</code> if you're only targeting modern browsers.

Example using <code>Symbol.toPrimitive</code>:

<pre><code>function sum(a) {
 let result = a;

 function f(b) {
 result += b;

 return f;
 }

 f[Symbol.toPrimitive] = hint =&gt; hint === 'string' ? String(result) : result;

 return f;
}
</code></pre>

Example using <code>.toString</code> and <code>.valueOf</code> pair.

<pre><code>function sum(a) {
 var result = a;

 function f(b) {
 result += b;

 return f;
 }

 // avoiding double conversion which will happen in case of .toString
 f.valueOf = function() { return result; };
 f.toString = function() { return String(result); };

 return f;
}
</code></pre>

Could any one explain how this function alert, when more no of brackets of parameters are passed. I could not able to understand it clearly.

<pre><code>function sum(a) {

 var sum = a

 function f(b) {
 sum += b
 return f
 }

 f.toString = function() { return sum }

 return f
}

alert( sum(1)(2) ) // 3
alert( sum(5)(-1)(2) ) // 6
alert( sum(6)(-1)(-2)(-3) ) // 0
alert( sum(0)(1)(2)(3)(4)(5) ) // 15
</code></pre>

Closure in Javascript with multiple brackets

有人能解释一下，当传递更多的参数时，这个函数是如何报警的吗？我不能清楚地理解它。function sum(a) {  var sum = a  function f(b) {    sum += b    return f  }  f.toString = function() { return sum }  retu...

问带有多个括号的Javascript中的闭包
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