使用for循环的`let`和block作用域的解释

本文介绍了使用for循环的`let`和block作用域的解释的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 我明白 let 阻止重复的声明是很好的。 让x; let x; //错误！ 也可以使用用 let 声明的变量在可以预期的关闭中 let i = 100; setTimeout（function（）{console.log（i）}，i）; //'100'后100 ms 我有一些困难抓住是如何 let 适用于循环。这似乎是针对循环的特定的。考虑经典问题： //打印'10'10次（var i = 0; i< ;（i; i ++）{process.nextTick（_ => console.log（i））} //打印'0'到'9' for（let i = 0; i& 10; i ++）{process.nextTick（_ => console.log（i））} 为什么在这个上下文中使用 let ？在我的想象中，尽管只有一个块可见，为实际上为每次迭代创建一个单独的块，并且 let 声明在该块内部完成...但是只有一个 let 声明来初始化该值。这只是ES6的语法糖吗？如何工作？ 我了解 var 和 let 并在上面说明了它们。我特别有兴趣了解为什么不同的声明导致使用循环的不同的输出。解决方案 不，更多比语法糖。血清细节埋在§13.6.3.9 CreatePerIterationEnvironment 。如果您在中使用让关键字语句，它将检查它绑定的名称，然后 创建一个新的词汇环境，其名称为a）初始化器表达式b）每次迭代（以前用于评估增量表达式） 将所有变量的值从一个值复制到下一个环境 desugars to a simple //在不引入块时省略大括号 var i; i = 0; if（i< 10） process.nextTick（_ => console.log（i）） i ++; if（i< 10） process.nextTick（_ => console.log（i）） i ++; ... 而 for（let i = 0; i< ; 10; i ++）{process.nextTick（_ => console.log（i））} 对更复杂的 desugar $ //使用缩进控制流 {let i; i = 0; __status = {i}; } {let {i} = __status; if（i< 10） process.nextTick（_ => console.log（i）） __status = {i}; } {let {i} = __status; i ++; if（i< 10） process.nextTick（_ => console.log（i）） __status = {i}; } {let {i} = __status; i ++; ... I understand that let prevents duplicate declarations which is nice.let x;let x; // error!Variables declared with let can also be used in closures which can be expectedlet i = 100;setTimeout(function () { console.log(i) }, i); // '100' after 100 msWhat I have a bit of difficulty grasping is how let applies to loops. This seems to be specific to for loops. Consider the classic problem:// prints '10' 10 timesfor (var i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }// prints '0' through '9'for (let i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) }Why does using let in this context work? In my imagination even though only one block is visible, for actually creates a separate block for each iteration and the let declaration is done inside of that block ... but there is only one let declaration to initialize the value. Is this just syntactic sugar for ES6? How is this working?I understand the differences between var and let and have illustrated them above. I'm particularly interested in understanding why the different declarations result in different output using a for loop. 解决方案 No, it's more than syntactic sugar. The gory details are buried in §13.6.3.9CreatePerIterationEnvironment.If you use that let keyword in the for statement, it will check what names it does bind and thencreate a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression)copy the values from all variables with those names from one to the next environmentYour loop statement for (var i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) } desugars to a simple// omitting braces when they don't introduce a blockvar i;i = 0;if (i < 10) process.nextTick(_ => console.log(i)) i++; if (i < 10) process.nextTick(_ => console.log(i)) i++; …while for (let i = 0; i < 10; i++) { process.nextTick(_ => console.log(i)) } does "desugar" to the much more complicated// using braces to explicitly denote block scopes,// using indentation for control flow{ let i; i = 0; __status = {i};}{ let {i} = __status; if (i < 10) process.nextTick(_ => console.log(i)) __status = {i};} { let {i} = __status; i++; if (i < 10) process.nextTick(_ => console.log(i)) __status = {i}; } { let {i} = __status; i++; … 这篇关于使用for循环的`let`和block作用域的解释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！