Typescript 中的 var 和 let 有什么区别?

问题描述

我提交了一个关于堆栈溢出的问题，询问如果之前的版本已经在执行，我该如何阻止 putTestQuestionResponses() 函数的执行.

I submitted a question on stack overflow asking how I could stop the putTestQuestionResponses() function from executing IF a previous version was already executing.

回复是在此代码的第 2 行中添加一个处理标志.

The reply was to add in a processing flag which is here on line 2 of this code.

你能告诉我为什么在这里使用let"而不是var"吗?

Can you tell me why use a "let" instead of a "var" here?

var promisePutTestQuestion;
let processing = false;

onEnter: ['$interval', 'questionService',
         ($interval, qus: IQuestionService) => {
  promisePutTestQuestion = $interval(() => {
     if (processing)
         return;
     processing = true;
     qus.putTestQuestionResponses()
     .then(() => processing = false)
  }, 5 * 1000);
}],
onExit: ['$interval', ($interval) => {
        $interval.cancel(promisePutTestQuestion);
}]

推荐答案

var 声明是函数范围的，而 let 声明是块范围的.

var declaration is function scoped and let declaration is block scoped.

参见 https://basarat.gitbooks.io/typescript/content/docs/let.html 了解更多详情.

See https://basarat.gitbooks.io/typescript/content/docs/let.html for more details.

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