javascript - 变量没有在 promise 中分配

Question

可能是我困了，但是在什么情况下会出现下面的情况呢？

let foo;

page
  .evaluate(() => {
    // return works... but not closure assignment

    // doesn't work
    foo = 'foo';

    // works
    return 'bar';
  })
  .then(bar => {
    console.log('foobar', foo, bar);
    // > foobar undefined bar
  });

这是在使用 puppeteer 的 mocha 测试中发生的

更新:完整的代码

节点 9.11.2

/* global describe, it, before, after */

const fs = require('fs-extra');
const path = require('path');
const assert = require('assert');
const puppeteer = require('puppeteer');
const sleep = require('shleep');

const extPath = path.resolve(__dirname, '..', 'build');
const { name } = fs.readJSONSync(path.resolve(extPath, 'manifest.json'));

// Access chrome object in Extensions
// https://github.com/GoogleChrome/puppeteer/issues/2878

describe('chrome extension', () => {
  let browser;
  let extensionPage;

  before(async function() {
    this.timeout(90 * 1000);

    // start puppeteer
    browser = await puppeteer.launch({
      headless: false,
      args: [
        `--disable-extensions-except=${extPath}`,
        `--load-extension=${extPath}`
      ]
    });

    // poll instead of hope this is enough time?
    const EXT_LOAD_DELAY = 100;
    await sleep(EXT_LOAD_DELAY);

    const targets = await browser.targets();

    const extensionTarget = targets.find(
      ({ _targetInfo }) =>
        _targetInfo.type === 'background_page' && _targetInfo.title === name
    );

    const page = await extensionTarget.page();

    let foo;

    page
      .evaluate(() => {
        // return works... but not closure assignment

        // doesn't work
        foo = 'foo';

        // doesn't log
        console.log('foo', foo);

        // works
        return 'bar';
      })
      .then(bar => {
        console.log('foobar', foo, bar);
        // > foobar undefined bar
      });
  });

  it('should load', async () => {
    assert(true);
  });
});

测试截图

Answer 1

puppeteer 中的 evaluate 方法没有您在代码中本地或全局声明的变量的概念。您传递给 evaluate 方法的函数是要在页面上下文(即在您的浏览器中)中执行的函数。由于 foo 没有在页面上下文中声明，它无法访问它，因此无法更新它的值。

因此逐步执行您的代码:

let foo;

await page.evaluate(() => {
  foo = 'foo';  // Since no variable foo is known to the evaluate method in the context of your page
                // this line , effectively, assigns a new variable called foo with a value 'foo' which
                // is then lost once the method has completed since its value is never returned.

  return 'bar'; // This value is returned from the method and is therefore retained below
})
.then(bar => {
  console.log('foobar', foo, bar);
  // foobar is output as expected since you've hardcoded it

  // foo is now referring to the global foo you declared earlier but since you have used `let` and not
  // assigned it any value, it is output as 'undefined'

  // bar is the value you returned from your 'evaluate' function and is therefore output as 'bar' as
  // expected in the console log.
});

如果您想使用evaluate 来更新您的变量foo，您必须这样做:

let foo;
foo = await page.evaluate(() => {
  return 'foo'
});
console.log(foo); // Now outputs 'foo' as expected

但是，您可以将变量注入(inject)evaluate 方法并更新它们的值(如果您愿意)，例如:

let foo = 'foo'
console.log(foo); // Outputs 'foo' as expected
foo = await page.evaluate((injectedFoo) => {
  return `new${injectedFoo}`;
}, foo);
console.log(foo); // Outputs 'newfoo'

那么这里发生的是，您通过将变量 foo 作为参数传递到方法声明的末尾将其注入(inject)到 evaluate 方法中。 evaluate 方法现在包含一个变量(为清楚起见，我将其称为 injectedFoo)，它带有 foo 变量的原始值。

然后我将附加到 foo 变量字符串开头的字符串 new 返回，并在控制台中输出它的最终值。

我希望这有助于解释 evaluate 方法的工作原理!