变量不分配内部承诺

Question

也许我很困,但在什么情况下会发生以下情况？

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
  });

这是在使用木偶操纵者的摩卡测试中发生的

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

evaluatepuppeteer 中的方法没有您在本地或全局代码中声明的变量概念。您传递给该evaluate方法的函数是要在页面上下文中（即在您的浏览器中）执行的函数。由于foo未在页面上下文中声明，因此无法访问它，因此无法更新其值。

因此，要逐步执行您的代码：

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.
});

如果你想使用evaluate来更新你的变量，foo你必须这样做：

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'

所以这里发生的事情是你通过在方法声明的末尾将变量作为参数传递来将变量foo注入到evaluate方法中。该evaluate方法现在包含一个变量（injectedFoo为了清楚起见，我已经调用了它），它带有foo变量的原始值。

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

我希望这有助于解释该evaluate方法的工作原理！