需要帮助理解JS代码 [英] need help understanding JS code
问题描述
您好我是javascript新手，我不明白以下代码：
var createAdders = function（）{
var fns = [];
for（var i = 1; i <4; i ++）{
fns [i] =（function（n）{
return i + n ;
}）;
}
return fns;
}
var adders = createAdders（）;
adders [1]（7）; // 11？
adders [2]（7）; // 11？
adders [3]（7）; // 11？
根据我的理解，7是作为参数传递的，但是 createAdders 不会将7赋给任何变量，所以这意味着7被传递给 createAdders（）中的下一个函数，即匿名函数和
上面的代码似乎工作，但对于每个调用，结果是11.
我发现这个代码在一个非常可靠的博客作为一个例子，一个情况下，闭包将上面的代码然后改变以下面的方式来引用闭包的优点。
var createAdders = function（）{
var fns = [];
for（var i = 1; i <4; i ++）{
（function（i）{
fns [i] =（function（n）{
return i + n;
}）;
}）（i）////为什么在这里使用（i）？它的目的是什么？////
}
return fns;
}
var adders = createAdders（）;
adders [1]（7）; // 8
adders [2]（7）; // 9
adders [3]（7）; // 10
这里是否也应用相同的逻辑？ p>
我需要了解如何为n分配值7
为什么（i）在新代码的函数结尾处使用
内部函数总是能够访问它们的外部函数变量，即使在外部函数完成执行之后。这个行为称为Closure。
在第一个片段中，你迭代超过3次，并在数组中推送函数。当循环完成执行你的变量i设置为4.并且您推入数组内的函数可以访问此变量。因此，无论在什么数组的索引执行函数，你总是得到11（4 + 7）。
在第二个片段中，你使用一个自执行匿名函数保持i的值。这个匿名函数会在每次交互时立即执行。因为在这个匿名函数中定义的闭包函数每次都引用i的新值。
所以你在数组中有一个三个函数，每个都有i （1,2,3）
编写第二个代码片段如下：
var createAdders = function {
var fns = [];
for（var i = 1; i <4; i ++）{
（function（a）{
fns [a] =（function（n）{
return a + n;
}）;
}）（i）
}
return fns;}
在javascript中，你可以有函数作为返回值。
和第二个数组可以存储函数里面的自己。
你在代码中做一个函数在数组中接受一个参数，然后调用它。
adders [1]（7）;
可按如下细分：
var fn = adders [1];
fn（7）; // thats how n is set to 7
Hi I am new to javascript and I cannot understand the following code:
var createAdders = function () {
var fns = [];
for (var i = 1; i < 4; i++) {
fns[i] = (function (n) {
return i + n;
});
}
return fns;
}
var adders = createAdders();
adders[1](7); //11 ??
adders[2](7); //11 ??
adders[3](7); //11 ??
From what I understand 7 is being passed as an argument but createAdders() doesn't assign 7 to any variable so does that mean 7 is being passed to the very next function within createAdders() i.e the anonymous function and assigned to the variable n.
Is my logic correct?
The code above does seem to work but for every call the result is 11. I found this code in a very reliable blog as an example to a situation where closures would be useful.The above code is then altered in the following way to cite the advantages of closures.
var createAdders = function () {
var fns = [];
for (var i = 1; i < 4; i++) {
(function (i) {
fns[i] = (function (n) {
return i + n;
});
})(i) //// why is (i) used here? what purpose does it serve??////
}
return fns;
}
var adders = createAdders();
adders[1](7); //8
adders[2](7); //9
adders[3](7); //10
Is the same logic applied here as well?
I need to understand how n is being assigned the value 7
Why is (i) being used at the end of the function in the new code?
In javascript , inner function always have access to their outer function variable even after the outer function is finished executing. This behaviour is called Closure.
In first snippet, you're iterating over 3 times and pushing the function inside an array.When the loop is finished executing your variable i is set to 4. And function you pushed inside the array has access to this variable . So no matter at what index of array you execute the function you always get 11 ( 4 + 7).
In Second snippet , you are using an self executing anonymous function to hold the value of i . This anonymous function gets immediately executed with every interation . Because of closure function defined inside this anonymous function has a reference to new value of i every time.
So you have a three function inside the array each with seperate value of i (1,2,3) Writing second snippet as follows makes it more clear
var createAdders = function(){
var fns = [ ];
for (var i=1; i<4; i++) {
(function(a) {
fns[a] = (function(n) {
return a+n;
});
})(i)
}
return fns;}
In javascript you can have function as return value. and secondly arrays can store function inside themself.
What your doing in code is pushing a function inside array which accepts a single argument and later invoking it.
adders[1](7);
can be broken down as follow
var fn = adders[1];
fn(7); //thats how n is set to 7
这篇关于需要帮助理解JS代码的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
