Javascript 对于在异步操作中未定义循环变量的类型

我想将数组中的数字按顺序写入HTML输入。my for..of循环中的数组值仅在异步操作中返回未定义的值。这是我的密码：

const puppeteer = require('puppeteer');
const years = [2000,2001,2002];

(async () => {
    const browser = await puppeteer.launch({
        executablePath: './chromium/chrome.exe',
        headless:false,
        product:'chrome',
        userDataDir: './cache'
    });
    const page = (await browser.pages())[0];
    await page.setViewport({width:1920, height:1080});
    await page.goto("https://www.immotop.lu/search/");

    const example = async ()=>  {
        for (year of years){
            await page.$eval('#search_form > div.filt-but-block > button.button-l3.filter-pan', el => el.click());
            await page.$eval('#year_build', el => el.value = "" + year) // year is undefined here
        }
    }
    example().then(() =>{
        console.log('done');
    })
})();

我认为这是由于在使用wait执行异步代码时通过数组进行同步循环，因此循环完成，变量在wait执行之前消失。此外，我听说应该有一种方法将变量单独保存在函数体中，但到目前为止，我还没有弄明白如何做

以下是错误消息：

(node:23492) UnhandledPromiseRejectionWarning: Error: Evaluation failed: 
ReferenceError: year is not defined
 (node:23492) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
    (node:23492) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

---

您尚未在for..of循环中定义迭代器变量，只需使用let关键字在for..of循环中定义“year”

这引发错误的原因可能是因为您可能正在将“Strict Mode”与“use Strict；”一起使用在.js文件的最顶端。因为在严格模式下，全局变量会为..in或for..类循环抛出异常

  for (let year of years){
                await page.$eval('#search_form > div.filt-but-block > button.button-l3.filter-pan', el => el.click());
                await page.$eval('#year_build', el => el.value = "" + year) // year is undefined here
                });
            }

---

当我们使用Puppeter时，有两个单独的javascript上下文：

节点上下文，脚本从中运行

浏览器上下文：在Chromium-Puppeter中工作的完全不同的javascript运行时只发送函数并接收它们的计算结果

浏览器中不存在在节点上下文中声明的变量和函数，反之亦然

// NODE CONTEXT
// this script works in Node.js
const puppeteer = require('puppeteer');
const years = [2000,2001,2002];

(async () => {
    const browser = await puppeteer.launch();
    const page = (await browser.pages())[0];
    await page.goto("https://www.immotop.lu/search/");

    for (year of years) { // <-- year exists in node.js context
        // But all functions executed inside of evaluation methods,
        // like page.evaluate, page.$eval, page.$$eval
        // are actually run in the browser
        // and there is no "year" variable there
        await page.$eval('#search_form > div.filt-but-block > button.button-l3.filter-pan', el => el.click());
        await page.$eval('#year_build', el => el.value = "" + year) // year is undefined here
    }
})();

args

就是我们要找的。这意味着在执行函数之后，我们可以根据需要传递尽可能多的附加参数。更正后的代码如下所示：

for (year of years) {
  const yearBuild = await page.$eval(
    "#year_build", // <-- element selector
    (el, yearFromNode) => (el.value = "" + yearFromNode), // function to run
    year // <-- variable from node context to send to the browser
  );
}

for（年份）{
const yearBuild=等待页面$eval(
“#year_build”，//（el.value=”“+yearFromNode），//要运行的函数
year//您错过了循环中的变量定义yiu必须在year之前使用let谢谢，确实修复了，但仍然存在相同的错误。谢谢，我现在使用了'let'，它仍然抛出相同的异常。据我所知，我没有使用严格模式，const puppeter=require（'puppeter'））；是我的原始代码中唯一缺少的一行。非常感谢！我现在非常理解这个问题。
for (year of years) {
  const yearBuild = await page.$eval(
    "#year_build", // <-- element selector
    (el, yearFromNode) => (el.value = "" + yearFromNode), // function to run
    year // <-- variable from node context to send to the browser
  );
}