blocks|key|4097419|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4097420|函数在什么时候知道每个元素|blockquote|4097421|4097422|4097423|任何时候(就像传入console.log时一样)。|offset|length|style|CODE|4097424|forEach函数在下面的行上调用它：|4097425|action(array[i]);|code-block|syntax|javascript|4097426|在这一点上，它只知道数组中的一个值，因为这是传入它的所有值。|4097427|(它还了解sum变量，因为该变量的定义范围比函数更广)。|4097428|entityMap^0|0|0|0|0|9|B|0|0|7|0|0|0|5|3|0^^$0|@$1|2|3|-4|4|5|6|Y|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|Z|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|10|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|11|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|12|7|@$H|13|I|14|J|K]]|8|@]|9|$]]|$1|L|3|M|4|5|6|15|7|@$H|16|I|17|J|K]]|8|@]|9|$]]|$1|N|3|O|4|P|6|18|7|@]|8|@]|9|$Q|R]]|$1|S|3|T|4|5|6|19|7|@]|8|@]|9|$]]|$1|U|3|V|4|5|6|1A|7|@$H|1B|I|1C|J|K]]|8|@]|9|$]]|$1|W|3|-4|4|5|6|1D|7|@]|8|@]|9|$]]]|X|$]]

<blockquote>
 at what point does the function know about every element
</blockquote>

At no point (just like when you pass in <code>console.log</code>).

The <code>forEach</code> function calls it on this line:

<pre><code>action(array[i]);
</code></pre>

At which point, it only knows about a single value from the array because that is all that is passed into it.

(It also knows about the <code>sum</code> variable because that is defined in a wider scope than the function).

blocks|key|1780116|text|在JavaScript中，函数是一等公民。这意味着它们可以被视为变量，就像字符串、数字等一样。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1780117|当你这样做的时候：|1780118|forEach(numbers,+function(number)+{
++sum+%2B=+number;
});|code-block|syntax|javascript|1780119|您正在向forEach传递一个匿名函数。函数不是在一个变量中，而是动态创建的。在您的forEach函数中，action将包含您的匿名函数。|offset|length|style|CODE|1780120|在for+for循环中，为每个元素调用action函数。|1780121|entityMap^0|0|0|0|4|7|16|7|1H|6|0|5|3|J|6|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@$K|W|L|X|M|N]|$K|Y|L|Z|M|N]|$K|10|L|11|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|12|8|@$K|13|L|14|M|N]|$K|15|L|16|M|N]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$]]

In JavaScript, functions are first-class citizens. That means they can be treated as variables, just like strings, numbers, etc.

When you do:

<pre><code>forEach(numbers, function(number) {
 sum += number;
});
</code></pre>

You are passing <code>forEach</code> an anonymous function. Instead of the function being in a variable, it's created on-the-fly. Inside your <code>forEach</code> function, <code>action</code> will contain your anonymous function.

In for <code>for</code> loop, the <code>action</code> function is called for each element.

blocks|key|1487120|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1487121|如何对其进行评估？|blockquote|1487122|1487123|1487124|它创建一个新的作用域(使用array、action和i变量)，并将函数分配给action变量-这是一个函数调用。|offset|length|style|CODE|1487125|你的|1487126|var+sum+=+0;
forEach([1,+2,+3,+4,+5],+function(number)+{
++sum+%2B=+number;
});|code-block|syntax|javascript|1487127|就像|1487128|var+sum+=+0;
{+//+let's+assume+block+scope+here
++var+array+=+[1,+2,+3,+4,+5],
++++++action+=+function(number)+{
++++++++sum+%2B=+number;
++++++},
++++++i;
++for+(i+=+0;+i+<+array.length;+i%2B%2B)
++++action(array[i]);
}|1487129|如果我把console.log传递给最后一个问题，它仍然需要求值，那么这不是也适用于整个函数吗，因为那个函数是参数？|1487130|1487131|是的，完全正确。您正在传递一个函数对象--无论是通过引用console.log变量还是通过动态创建它(使用函数表达式)来获取对象，对forEach来说都无关紧要。它只会在action(…)中执行--+array[i]值是为你的函数的number参数传递的。|1487132|entityMap^0|0|0|0|0|D|5|J|6|Q|1|12|6|0|0|0|0|0|4|B|0|0|S|B|1U|7|2D|9|2S|8|38|6|0^^$0|@$1|2|3|-4|4|5|6|13|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|14|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|15|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|16|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|17|7|@$H|18|I|19|J|K]|$H|1A|I|1B|J|K]|$H|1C|I|1D|J|K]|$H|1E|I|1F|J|K]]|8|@]|9|$]]|$1|L|3|M|4|5|6|1G|7|@]|8|@]|9|$]]|$1|N|3|O|4|P|6|1H|7|@]|8|@]|9|$Q|R]]|$1|S|3|T|4|5|6|1I|7|@]|8|@]|9|$]]|$1|U|3|V|4|P|6|1J|7|@]|8|@]|9|$Q|R]]|$1|W|3|X|4|5|6|1K|7|@$H|1L|I|1M|J|K]]|8|@]|9|$]]|$1|Y|3|-4|4|5|6|1N|7|@]|8|@]|9|$]]|$1|Z|3|10|4|5|6|1O|7|@$H|1P|I|1Q|J|K]|$H|1R|I|1S|J|K]|$H|1T|I|1U|J|K]|$H|1V|I|1W|J|K]|$H|1X|I|1Y|J|K]]|8|@]|9|$]]|$1|11|3|-4|4|5|6|1Z|7|@]|8|@]|9|$]]]|12|$]]

<blockquote>
 How does it get evaluated?
</blockquote>

It creates a new scope (with <code>array</code>, <code>action</code> and <code>i</code> variables) and assigns the function to the <code>action</code> variable - that's a function invocation.

Your

<pre><code>var sum = 0;
forEach([1, 2, 3, 4, 5], function(number) {
 sum += number;
});
</code></pre>

is just the same as

<pre><code>var sum = 0;
{ // let's assume block scope here
 var array = [1, 2, 3, 4, 5],
 action = function(number) {
 sum += number;
 },
 i;
 for (i = 0; i &lt; array.length; i++)
 action(array[i]);
}
</code></pre>

<blockquote>
 If I pass <code>console.log</code> into the last problem and it still has to be evaluated, then doesn't that apply to the entire function too since that function is the parameter?
</blockquote>

Yes, exactly. You are passing a function object - and whether get that by referencing the <code>console.log</code> variable or by creating it on the fly (with the function expression) doesn't matter to <code>forEach</code>. It only will get executed with <code>action(…)</code> - where the <code>array[i]</code> value is passed for the <code>number</code> parameter of your function.

blocks|key|1780216|text|以下是您的问题的解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1780217|++++function+forEach(array,+action)+{
++++++++for+(var+i+=+0;+i+<+array.length;+i%2B%2B){
++++++++++++if(typeof+action+==+'function'){
++++++++++++++++action(array[i]);
++++++++++++}else{
++++++++++++++++var+slices+=+action.match(/(.%2B)\.([a-zA-Z0-9]%2B)/);
++++++++++++++++var+object+=+eval(slices[1]);
++++++++++++++++var+action+=+slices[2];
++++++++++++++++object[action](array[i]);
++++++++++++}
++++++++}
++++}|code-block|syntax|javascript|1780218|我对这两种情况都进行了测试，它像魔术一样工作|1780219|​|1780220|📷|atomic|offset|length|1780221|1780222|1780223|1780224|1780225|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/a7c927bc1206f60dce415d058f06b61e.png|imageAlt|1|https://ask.qcloudimg.com/http-save/yehe-900000/8ebd98c5136b413caf3dfa7dbcbde790.png^0|0|0|0|0|0|1|0|0|0|0|0|1|1|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|16|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|17|8|@]|9|@]|A|$]]|$1|K|3|L|5|M|7|18|8|@]|9|@$N|19|O|1A|1|1B]]|A|$]]|$1|P|3|J|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|J|5|6|7|1D|8|@]|9|@]|A|$]]|$1|R|3|L|5|M|7|1E|8|@]|9|@$N|1F|O|1G|1|1H]]|A|$]]|$1|S|3|J|5|6|7|1I|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10|11|-4]]|12|$5|W|X|Y|A|$Z|13|11|-4]]]]

Heres the solution to your problems:

<pre><code> function forEach(array, action) {
 for (var i = 0; i &lt; array.length; i++){
 if(typeof action == 'function'){
 action(array[i]);
 }else{
 var slices = action.match(/(.+)\.([a-zA-Z0-9]+)/);
 var object = eval(slices[1]);
 var action = slices[2];
 object[action](array[i]);
 }
 }
 }
</code></pre>

I've tested it with both scenarios and it works like magic

<img src="https://i.stack.imgur.com/6oC8p.png" alt="enter image description here">

<img src="https://i.stack.imgur.com/JzpHI.png" alt="enter image description here">

I have a forEach function defined to do "something" with all the items in an array:

<pre><code>function forEach(array, action) {
 for (var i = 0; i &lt; array.length; i++)
 action(array[i]);
} 

var numbers = [1, 2, 3, 4, 5], sum = 0;
</code></pre>

So I could do:

<pre><code>forEach(numbers, console.log);
</code></pre>

And it would print out all the numbers because 'console.log(array[i])' prints the number to the console.
I get that part.

Here's where I'm stuck: If I pass the function below into the place of the action parameter, instead of 'console.log' then at what point does the function know about every element? 

<pre><code>forEach(numbers, function(number) {
 sum += number;
});

console.log(sum);
// 15
</code></pre>

How does it get evaluated? If I pass console.log into the last problem and it still has to be evaluated as 'console.log(array[i])' with the '(array[i])' code next to whatever parameter is being passed in, then doesn't that get applied to the entire function too since that function is the parameter? Such as below:

<pre><code>function(number) { sum += number; }(array[i])
</code></pre>

How does a function gets passed in to another function?

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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我定义了一个forEach函数，用于对数组中的所有项执行“某些操作”：function forEach(array, action) { for (var i = 0; i < array.length; i++) action(array[i]);} var numbers = [1, 2, 3, 4, 5], ...

问一个函数是如何传递给另一个函数的？
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问一个函数是如何传递给另一个函数的？EN