I want to find average color of all images in the page. I am using Puppeteer to launch chromium and then run javascript inside browser.

Problem is that I am using Promies.all to asynchronously calculate all colors inside browser.

I want to return calculated image colors to page.evaluate. How can I do that? Is it possible?

Following code does not work, because imgcolors is no defined obviously, it is returned inside Promise.all callback.

   const returned = await page.evaluate(async () => {
   const fac = new FastAverageColor();
   const imgs = Array.from(document.querySelectorAll('img'));
   const imgpromises = imgs.map(img => fac.getColorAsync(img.src));


   Promise.all(imgpromises).then(imgcolors => { return imgcolors;});

   return {
     document:{
       height: document.body.scrollHeight, 
       width:document.body.scrollWidth 
     },
     imgcolors:imgcolors
   };
 });

I have tried following:

Promise.all(imgpromises).then(imgcolors => { 
      
      return {
        elements:elements, 
        document:{
          height: document.body.scrollHeight, 
          width:document.body.scrollWidth 
        },
        imgcolors:imgcolors
      };
    });

Now I am not returing data to page.evaluate puppeteer function. So const returned is undefined.

How to do that? Thank you in advance!

I want to find average color of all images in the page. I am using Puppeteer to launch chromium and then run javascript inside browser.

Problem is that I am using Promies.all to asynchronously calculate all colors inside browser.

I want to return calculated image colors to page.evaluate. How can I do that? Is it possible?

Following code does not work, because imgcolors is no defined obviously, it is returned inside Promise.all callback.

   const returned = await page.evaluate(async () => {
   const fac = new FastAverageColor();
   const imgs = Array.from(document.querySelectorAll('img'));
   const imgpromises = imgs.map(img => fac.getColorAsync(img.src));


   Promise.all(imgpromises).then(imgcolors => { return imgcolors;});

   return {
     document:{
       height: document.body.scrollHeight, 
       width:document.body.scrollWidth 
     },
     imgcolors:imgcolors
   };
 });

I have tried following:

Promise.all(imgpromises).then(imgcolors => { 
      
      return {
        elements:elements, 
        document:{
          height: document.body.scrollHeight, 
          width:document.body.scrollWidth 
        },
        imgcolors:imgcolors
      };
    });

Now I am not returing data to page.evaluate puppeteer function. So const returned is undefined.

How to do that? Thank you in advance!

• javascript
• node.js
• asynchronous
• promise
• puppeteer

1 Answer 1

In the page.evaluate function, you can return a promise, which will resolve to a value. In this case Promise.all returns a promise, which you can chain to return the value you want. It'd look something like this:

const returned = await page.evaluate(() => {
  const fac = new FastAverageColor();
  const imgs = Array.from(document.querySelectorAll('img'));
  const imgpromises = imgs.map(img => fac.getColorAsync(img.src));
  return Promise.all(imgpromises).then(imgcolors => { 
    return {
      document:{
        height: document.body.scrollHeight, 
        width: document.body.scrollWidth 
      },
      imgcolors: imgcolors
    };
  });
});

Alternatively, since you are using async functions, you can just use the await operator to wait for values to be resolved:

const returned = await page.evaluate(async () => {
  const fac = new FastAverageColor();
  const imgs = Array.from(document.querySelectorAll('img'));
  const imgpromises = imgs.map(img => fac.getColorAsync(img.src));
  const imgcolors = await Promise.all(imgpromises);
  return {
    document:{
      height: document.body.scrollHeight, 
      width: document.body.scrollWidth 
    },
    imgcolors: imgcolors
  };
});

Note that page.evaluate can only return serializable values, i.e. things that can be JSON.stringify'd. It looks like imgcolors should be serializable, but in other cases, you might need to convert the values to something serializable.