Javascript jQuery 的每个方法中的“callback.call( value, i, value )”是什么意思？
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What is the meaning of "callback.call( value, i, value )" in jQuery's each method?
提问by Mert Nuhoglu
each()method in jQuery contains such a statement:
each()jQuery 中的方法包含这样的语句：
callback.call( value, i, value )
I couldn't understand what this statement means exactly.
我无法理解这句话的确切含义。
I know what callbackand callmean but I couldn't get the arguments of the function call: (value,i,value). What does this mean?
我知道callback和call的意思，但我不能让函数调用的参数：(value,i,value)。这是什么意思？
The statement is used in a for block of each()but my question is independent of that context.
该语句用于 for 块中，each()但我的问题与该上下文无关。
from the jQuery source:
来自 jQuery 源代码：
for ( var value = object[0];
i < length &&
callback.call( value, i, value ) // <=== LOOK!
!== false;
value = object[++i] ) {}
回答by lonesomeday
The callmethod exists on all functions in Javascript. It allows you to call the function and in doing so set the value of thiswithin that function.
该call方法存在于 Javascript 中的所有函数中。它允许您调用该函数并在此过程中设置该this函数内的值。
function myFunc() {
console.log(this);
}
myFunc.call(document.body);
In this example, thiswithin myFuncwill be document.body.
在此示例中，this内myFunc将是document.body。
The first parameter of callis the value to be set as this; subsequent parameters are passed on to the function as normal parameters. So, in your example:
的第一个参数call是要设置为的值this；后续参数作为普通参数传递给函数。所以，在你的例子中：
callback.call( value, i, value )
this is equivalent to
这相当于
callback(i, value)
except that, within the callback, thisis now also set to value.
除此之外，在回调中，this现在也设置为value。
回答by Pointy
The .each()method calls the callback you pass it with the element (current iteration "target") as both the context object (the value of this) and as the second parameter.
该.each()方法调用您通过元素（当前迭代“目标”）作为上下文对象（ 的值this）和第二个参数传递给它的回调。
Thus, in one of those functions:
因此，在这些功能之一中：
$('.foo').each(function(i, elem) {
var $this = $(this), $elem = $(elem);
The variables $thisand $elemare interchangeable.
变量$this和$elem可以互换。
The first argument to .call()is the value to which thisshould be bound, if that wasn't clear. The rest of the arguments to .call()are just passed as plain arguments to the function.
如果不清楚，第一个参数.call()是this应该绑定到的值。其余的参数.call()只是作为普通参数传递给函数。
回答by SLaks
This calls the callbackmethod with thisset to value(the first parameter to call) and with the arguments iand value. (The other parameters to call)
这将调用callback具有thisset to value（第一个参数 to call）和参数iand 的方法value。（其他参数为call）
