Await 仅在 nodejs 的异步函数中有效

Question

I'm developing a web capture app and it seems that all the functions are with async but console shows me a SyntaxError: await is only valid in async function error.

I tried to change all the functions to async but it seems still not working. Is it an error because of the fs module? I think when I tried without fs module in another app it actually works.

Here's my full code

const puppeteer = require("puppeteer");
const fs = require('fs');

let galleryName = "frozen"; // Enter gallery name

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  // Adjustments particular to this page to ensure we hit desktop breakpoint.
  page.setViewport({
    width: 1000,
    height: 10000,
    deviceScaleFactor: 1
  });

  fs.readFile('db.txt', function (err, data) {
    if (err) throw err;
    let array = data.toString().split("\n");
    for (i in array) {
      console.log(`Now Processing : ${array[i]} ${array.length - i -1} left`);
      await page.goto(`https://gall.dcinside.com/${galleryName}/${array[i]}`), { // !!!!ERROR shows from here
        waitUntil: "networkidle2",
        timeout: 0
      };
      async function screenshotDOMElement(opts = {}) {
        const padding = "padding" in opts ? opts.padding : 0;
        const path = "path" in opts ? opts.path : null;
        const selector = opts.selector;

        if (!selector) throw Error("Please provide a selector.");

        const rect = await page.evaluate(selector => {
          const element = document.querySelector(selector);
          if (!element) return null;
          const {
            x,
            y,
            width,
            height
          } = element.getBoundingClientRect();
          return {
            left: x,
            top: y,
            width,
            height,
            id: element.id
          };
        }, selector);

        if (!rect)
          throw Error(
            `Could not find element that matches selector: ${selector}.`
          );

        return await page.screenshot({
          path,
          clip: {
            x: rect.left - padding,
            y: rect.top - padding,
            width: rect.width,
            height: rect.height + padding * 2
          }
        });
      }

      await screenshotDOMElement({
        path: `./result/${pageNumArray[i]}.png`,
        selector: ".view_content_wrap",
        padding: 10
      });
    }
  });
  //   // await browser.close();
})();

Answer 1

fs 中的回调函数也需要异步，

fs.readFile('db.txt', async function (err, data) {}

Answer 2

The answers suggesting to make the callback async will make the error disappear, but the overall readFile operation will not be awaited. So, if for instance, you would enable the line with await browser.close(); , that browser.close() will run before the readFile callback executes.

When you use promises, you are better off using the Promise API for fs :

const fsPromises = require('fs').promises;

const handle = await fs.readFile('db.txt');
const data = await handle.readFile();
let array = data.toString().split("\n");
// ...etc


await handle.close();

This way:

• You don't have a callback, and you can use await with the file and other asynchronous operations
• The main async function's returned promise will only resolve when all await -tasks have been performed.

Answer 3

您必须在回调函数中使用async才能使用await否则您会收到错误消息。

fs.readFile('db.txt', async (err, data) => { });

Answer 4

the callback function inside fs needs to be async as well

this part : fs.readFile('db.txt', async function (err, data)

const puppeteer = require("puppeteer");
const fs = require('fs');

let galleryName = "frozen"; // Enter gallery name

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  // Adjustments particular to this page to ensure we hit desktop breakpoint.
  page.setViewport({
    width: 1000,
    height: 10000,
    deviceScaleFactor: 1
  });

  fs.readFile('db.txt', async function (err, data) {
    if (err) throw err;
    let array = data.toString().split("\n");
    for (i in array) {
      console.log(`Now Processing : ${array[i]} ${array.length - i -1} left`);
      await page.goto(`https://gall.dcinside.com/${galleryName}/${array[i]}`), { // !!!!ERROR shows from here
        waitUntil: "networkidle2",
        timeout: 0
      };
      async function screenshotDOMElement(opts = {}) {
        const padding = "padding" in opts ? opts.padding : 0;
        const path = "path" in opts ? opts.path : null;
        const selector = opts.selector;

        if (!selector) throw Error("Please provide a selector.");

        const rect = await page.evaluate(selector => {
          const element = document.querySelector(selector);
          if (!element) return null;
          const {
            x,
            y,
            width,
            height
          } = element.getBoundingClientRect();
          return {
            left: x,
            top: y,
            width,
            height,
            id: element.id
          };
        }, selector);

        if (!rect)
          throw Error(
            `Could not find element that matches selector: ${selector}.`
          );

        return await page.screenshot({
          path,
          clip: {
            x: rect.left - padding,
            y: rect.top - padding,
            width: rect.width,
            height: rect.height + padding * 2
          }
        });
      }

      await screenshotDOMElement({
        path: `./result/${pageNumArray[i]}.png`,
        selector: ".view_content_wrap",
        padding: 10
      });
    }
  });
  //   // await browser.close();
})();

read more here about async-await ' https://javascript.info/async-await