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title | localeTitle |
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Arguments | 参数 |
arguments对象是一个类似于数组的对象 (因为 对象 的结构类似于数组的结构,但它不应该被认为是一个数组,因为它具有对象的所有功能) ,它存储了你的所有参数传递给函数,特别是该函数专有。如果要将3个参数传递给函数,比如storeNames()
,那么这三个参数将存储在一个名为arguments的对象中,当我们传递参数storeNames("Mulder", "Scully", "Alex Krycek")
它将看起来像这样storeNames("Mulder", "Scully", "Alex Krycek")
我们的功能:
- 首先,我们声明一个函数并使其返回arguments对象。
```的JavaScript
function storeNames(){return arguments; }
* Then, when we execute that function with **n arguments**, 3 in this case, it will return the object to us and it will **look like** an array. We can convert it to an array, but more on that later...
JavaScript的 //如果我们在控制台中执行以下行: storeNames(“Mulder”,“Scully”,“Alex Kryceck”); //输出为{'0':'Mulder','1':'Scully','2':'Alex Kryceck'}
If you want to know more about this, such as converting it to an array or the optimization problem that comes with using the _slice(_) method and how to solve it, click on **read more** (Gitter Chat Only).
## Treat it as an array
You can invoke arguments by using `arguments[n]` (where _n_ is the index of the argument in the array-like object) but if you want to use it as an array for iteration purposes or applying array methods to it, you need to _convert it to an array_ by declaring a variable and using the Array.prototype.slice.call method (because _arguments_ is not an array):
JavaScript的 var args = Array.prototype.slice.call(arguments);
//或es6方式: var args = Array.from(arguments)
Since **slice()** has two (the parameter **end** is optional) parameters, you can grab a certain portion of the arguments by specifying (using the _slice.call()_ method renders these two parameters optional, not just _end_) the beginning and the ending of your portion; check out the following code:
JavaScript的 function getGrades(){ var args = Array.prototype.slice.call(arguments,1,3); 回归 }
//让我们输出这个! console.log(getGrades(90,100,75,40,89,95));
//输出应该是:// // [100,75] < - 为什么?因为它从索引1开始并在索引3处停止 //所以,没有考虑索引3(40)。 // //如果我们删除'3'参数,只留下(参数,1)我们得到 //来自索引1的每个参数:[100,75,40,89,95]。
### Optimization issues with Array.slice()
There is a little problem, it's not recommended to use slice in the arguments object (optimization reasons)...
> **Important**: You should not slice on arguments because it prevents optimizations in JavaScript engines (V8 for example). Instead, try constructing a new array by iterating through the arguments object.
>
> _by_ **_Mozilla Developer Network_** <a href='https://developer.mozilla.org/ca/docs/Web/JavaScript/Reference/Functions/arguments' target='_blank' rel='nofollow'>(reference)<a>
So, what other method is available to convert _arguments_ to an array? I recommend the for-loop (not the for-in loop), you can do it like this:
JavaScript的 var args = []; //首先是空数组。 for(var i = 0; i <arguments.length; i ++){ args.push(参数[I]) } //现在'args'是一个包含你的参数的数组。 ```
有关优化问题的更多信息:
优化杀手: 管理参数
ES6 rest参数作为绕过arguments对象的一种方式
在ES2015 / ES6中,可以在大多数地方使用rest参数( ...
)而不是arguments对象。假设我们有以下功能(非ES6):
function getIntoAnArgument() {
var args = arguments.slice();
args.forEach(function(arg) {
console.log(arg);
});
}
该功能可以在ES6中替换为:
function getIntoAnArgument(...args) {
args.forEach(arg => console.log(arg));
}
请注意,我们还使用了箭头函数来缩短forEach回调!
参数对象在箭头函数的主体内不可用。
rest参数必须始终作为函数定义中的最后一个参数。
function getIntoAnArgument(arg1, arg2, arg3, ...restOfArgs /*no more arguments allowed here*/) { //function body }