How get all computed css properties of element and its children in Javascript

Question

I'm trying to set up a service in python using pdfKit to create a pdf file from html files.

So basically I will send my element as string and expect the server to return a pdf version of it, but to be an accurate representation I also need to send a css file of the element.

How can I do this? Generate JSON / object with only the relevant style properties and selectors of an element and all its children. Respecting hierarchy and no duplicates. There are similar questions but they are outdated and tend to not consider children elements.

I was thinking maybe there is a way to create a new DOM from this element and then get the root css?

Answer 1

Here is something I came up with, basically pass the element you want to extract the styles of and ones of its children, and it will return you the stylesheet as a string. Open your console before running the snippet and you will see the output from the console.log.

Because I wanted to support the extraction of every element even those without a selector, I had to replace each element id by a unique uuid specifically generated for them in order to facilitate the styling of your output. The problem with this approach is in case you are using ids for styling or for user interaction, you are going to loose such functionality on concerned elements after calling extractCSS.

However, it is pretty trivial to use the oldId I'm passing to change back once your pdfKit process finished the generation. Simply call swapBackIds passing the elements returned by the function. You can see the difference of behavior if you uncomment the call in my snippet: the #root pink background would disappear because the styling targets an element id.

All in all, you need to:

1. Call extractCSS with the element you want to extract
2. Generate your pdf using res.stylesheet
3. Call swapBackIds with res.elements

// Generate an unique id for your element
// From https://stackoverflow.com/a/2117523/2054072
function uuidv4 () {
  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
    var r = Math.random() * 16 | 0, v = c == 'x' ? r : (r & 0x3 | 0x8);
    return v.toString(16);
  });
}

// Flatten an array
// https://stackoverflow.com/a/15030117/2054072
function flatten(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}

function recursiveExtract (element) {
  var id = uuidv4()
  var oldId = element.id

  var computed = window.getComputedStyle(element)
  var style = computed.cssText

  // Now that we get the style, we can swap the id
  element.setAttribute('id', id)

  // The children are not a real array but a NodeList, we need to convert them
  // so we can map over them easily
  var children = Array.prototype.slice.call(element.children)
  return [{ id: id, style: style, oldId: oldId }].concat(children.map(recursiveExtract))
}

function extractCSS (element) {
  if (!element) { return { elements: [], stylesheet: '' } }

  var raw = recursiveExtract(element)
  var flat = flatten(raw)
  
  return {
    elements: flat,
    stylesheet: flat.reduce(function (acc, cur) {
      var style = '#' + cur.id + ' {\n' + cur.style + '\n}\n\n'
      return acc + style
    }, '')
  }
}

var pdfElement = document.querySelector('#root')
var res = extractCSS(pdfElement)

console.log(res.stylesheet)

function swapBackIds (elements) {
  elements.forEach(function (e) {
    var element = document.getElementById(e.id)
    element.setAttribute('id', e.oldId)
  })
}

swapBackIds(res.elements)

#root {
  background-color: pink;
}

.style-from-class {
  background-color: red;
  width: 200px;
  height: 200px;
}

.style-from-id {
  background-color: green;
  width: 100px;
  height: 100px;
}

<div id="root">
  <span>normal</span>
  <span style="background: blue">inline</span>
  <div class="style-from-class">
    style-class
  </div>
  <div class="style-from-id">
    style-id
    <div style="font-size: 10px">a very nested</div>
    <div style="font-size: 12px; color: white">and another</div>
  </div>
</div>

<div id="ignored-sibling">
</div>

Answer 2

 
let para = document.querySelector('p');
let compStyles = window.getComputedStyle(para);
para.textContent = 'My computed font-size is ' + compStyles.getPropertyValue('font-size') + ',\nand my computed background is ' +  compStyles.getPropertyValue('background') + '.';

p {
  width: 400px;
  margin: 0 auto;
  padding: 20px;
  font: 2rem/2 sans-serif;
  text-align: center;
  background: purple;
  color: white;
}

<p>Hello</p>

you can use getComputedStyle method to get computed value of style property