JavaScript的“with with”是否合法用途？声明？ [英] Are there legitimate uses for JavaScript's "with" statement?

问题描述

Alan Storm的评论以回应我对 with statement 让我思考。我很少找到使用这种特定语言功能的理由，并且从未考虑过如何引起麻烦。现在，我很好奇如何有效地使用和，同时避免陷阱。

你在哪里找到和语句有用吗？

解决方案

发生了另一种用途今天对我来说，所以我兴奋地搜索了网络，发现现在已经提到它了：在块范围内定义变量。

背景

<尽管JavaScript与C和C ++表面相似，但它并没有将变量范围限定在它们定义的块中：

  var name =Joe; 
 if（true）
 {
 var name =Jack; 
} 
 //名称现在包含Jack
  

声明关闭循环是一个常见的任务，这可能导致错误：

  for（var i = 0; i< 3; ++ i）
 {
 var num = i; 
 setTimeout（function（）{alert（num）;}，10）; 
} 
  

因为for循环没有引入新范围，所以 num - 值为 2 - 将由所有三个函数共享。

新范围：让和与

在 let 语句-US / docs / Web / JavaScript / Reference / Statements / letrel =noreferrer> ES6 ，在必要时引入新范围以避免这些问题变得容易：

  //在此语句中引入的变量
 //的范围限定为循环的每次迭代
 for（let i = 0; i< 3; ++ i）
 {
 setTimeout（function（）{alert（i）;}，10）; 
} 
  

甚至：

  for（var i = 0; i< 3; ++ i）
 {
 //此语句中引入的变量
 //是作用域的到包含它的块。 
 let num = i; 
 setTimeout（function（）{alert（num）;}，10）; 
} 
  

在ES6普遍可用之前，此用途仅限于最新的浏览器和开发人员愿意使用转运。但是，我们可以使用与轻松模拟此行为：

  for（ var i = 0; i< 3; ++ i）
 {
 //在此语句中引入的对象成员
 //作用于其后的块。 
 with（{num：i}）
 {
 setTimeout（function（）{alert（num）;}，10）; 
} 
} 
  

循环现在按预期工作，创建三个单独的变量值为0到2.请注意，在块中声明的变量不限于它，与C ++中块的行为不同（在C中，变量必须在块的开头声明，所以在某种程度上它是相似的）。这种行为实际上非常类似于 <$ c在早期版本的Mozilla浏览器中引入了$ c> let 块语法，但在其他地方没有广泛采用。

Alan Storm's comments in response to my answer regarding the with statement got me thinking. I've seldom found a reason to use this particular language feature, and had never given much thought to how it might cause trouble. Now, I'm curious as to how I might make effective use of with, while avoiding its pitfalls.

Where have you found the with statement useful?

解决方案

Another use occurred to me today, so I searched the web excitedly and found an existing mention of it: Defining Variables inside Block Scope.

Background

JavaScript, in spite of its superficial resemblance to C and C++, does not scope variables to the block they are defined in:

var name = "Joe";
if ( true )
{
   var name = "Jack";
}
// name now contains "Jack"

Declaring a closure in a loop is a common task where this can lead to errors:

for (var i=0; i<3; ++i)
{
   var num = i;
   setTimeout(function() { alert(num); }, 10);
}

Because the for loop does not introduce a new scope, the same num - with a value of 2 - will be shared by all three functions.

A new scope: let and with

With the introduction of the let statement in ES6, it becomes easy to introduce a new scope when necessary to avoid these problems:

// variables introduced in this statement 
// are scoped to each iteration of the loop
for (let i=0; i<3; ++i)
{
   setTimeout(function() { alert(i); }, 10);
}

Or even:

for (var i=0; i<3; ++i)
{
   // variables introduced in this statement 
   // are scoped to the block containing it.
   let num = i;
   setTimeout(function() { alert(num); }, 10);
}

Until ES6 is universally available, this use remains limited to the newest browsers and developers willing to use transpilers. However, we can easily simulate this behavior using with:

for (var i=0; i<3; ++i)
{
   // object members introduced in this statement 
   // are scoped to the block following it.
   with ({num: i})
   {
      setTimeout(function() { alert(num); }, 10);
   }
}

The loop now works as intended, creating three separate variables with values from 0 to 2. Note that variables declared within the block are not scoped to it, unlike the behavior of blocks in C++ (in C, variables must be declared at the start of a block, so in a way it is similar). This behavior is actually quite similar to a let block syntax introduced in earlier versions of Mozilla browsers, but not widely adopted elsewhere.

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