'var'和'let'in Typescipt 1.5 [英] 'var' and 'let' in Typescipt 1.5

问题描述

在Typescript中使用 var 或 let 之间的区别是什么？我知道'let'允许变量被进一步定义到一个范围，而不是在所述范围之外使用。这显然是一个很好的优势，迭代器在for循环。我知道这是一个ES6的定义，所以编译到ES6应该看起来几乎相同的'var'和'let'。

What exactly is the difference between using either 'var' or 'let' in Typescript? I know that 'let' allows the variable to be defined further into a scope without it being used outside of said scope. That is obviously a nice advantage for iterators in for loops. I know this is an ES6 definition, so compiling to ES6 should look nearly identical in terms of 'var' and 'let'.

但是，当它编译成ES5 javascript不支持'let'，发生什么？是否有奇怪命名的变量，编译器创建，以防止在Typescript文件的上下文中使用所述变量？在整个函数中定义许多let变量的情况下会发生什么？

But, when it compiles down into ES5 javascript which doesn't support 'let', what exactly is happening? Are there oddly named variables that the compiler creates so that it prevents using said variables in the context of the Typescript file? What happens in situations where you define many 'let' variables throughout a function? Is there a performance impact I should be concerned about?

基本上，当类型脚本编译让变量到ES5 javascript时会发生什么？

Basically, what is happening when typescript compiles let variables to ES5 javascript?

推荐答案

最简单的方法是找到这些类型的问题的答案，在TypeScript Playground 。

The easiest way to find answers to these type of questions to try it out in the TypeScript Playground.

TypeScript重命名块范围内的变量。与使用 var 定义的常规变量相比，没有性能开销。

TypeScript renames the variables within the block scope. There's no performance overhead compared to using regular variables defined with var.

此代码：

var i = 0;
for (let i = 0; i < 5; i++) {}
i++;

编译为：

var i = 0;
for (var i_1 = 0; i_1 < 5; i_1++) { }
i++;

这里是关于 let 的一些更多信息。

Here is some more information regarding let.

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