79 lines
4.4 KiB
Markdown
79 lines
4.4 KiB
Markdown
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---
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title: Arguments
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localeTitle: 参数
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---
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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：
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* 首先，我们声明一个函数并使其返回arguments对象。
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\`\`\`的JavaScript
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function storeNames（）{return arguments; }
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```
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* 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...
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```
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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'}
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```
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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).
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## Treat it as an array
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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):
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```
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JavaScript的 var args = Array.prototype.slice.call（arguments）;
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//或es6方式： var args = Array.from（arguments）
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```
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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:
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```
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JavaScript的 function getGrades（）{ var args = Array.prototype.slice.call（arguments，1,3）; 回归 }
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//让我们输出这个！ console.log（getGrades（90,100,75,40,89,95））;
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//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：\[100,75,40,89,95\]。
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```
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### Optimization issues with Array.slice()
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There is a little problem, it's not recommended to use slice in the arguments object (optimization reasons)...
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> **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.
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>
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> _by_ **_Mozilla Developer Network_** <a href='https://developer.mozilla.org/ca/docs/Web/JavaScript/Reference/Functions/arguments' target='_blank' rel='nofollow'>(reference)<a>
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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:
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```
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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。 \`\`\`
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有关优化问题的更多信息：
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优化杀手： [管理参数](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments)
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### ES6 rest参数作为绕过arguments对象的一种方式
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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）：
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```
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function getIntoAnArgument() {
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var args = arguments.slice();
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args.forEach(function(arg) {
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console.log(arg);
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});
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}
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```
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该功能可以在ES6中替换为：
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```
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function getIntoAnArgument(...args) {
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args.forEach(arg => console.log(arg));
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}
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```
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请注意，我们还使用了箭头函数来缩短forEach回调！
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参数对象在箭头函数的主体内不可用。
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rest参数必须始终作为函数定义中的最后一个参数。
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`function getIntoAnArgument(arg1, arg2, arg3, ...restOfArgs /*no more arguments allowed here*/) { //function body }`
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