Find all CSS rules that apply to an element

Question

Many tools/APIs provide ways of selecting elements of specific classes or IDs. There\'s also possible to inspect the raw stylesheets loaded by the browser.

However,

Answer 1

Here is my version of getMatchedCSSRules function which support @media query.

const getMatchedCSSRules = (el) => {
  let rules = [...document.styleSheets]
  rules = rules.filter(({ href }) => !href)
  rules = rules.map((sheet) => [...(sheet.cssRules || sheet.rules || [])].map((rule) => {
    if (rule instanceof CSSStyleRule) {
      return [rule]
    } else if (rule instanceof CSSMediaRule && window.matchMedia(rule.conditionText)) {
      return [...rule.cssRules]
    }
    return []
  }))
  rules = rules.reduce((acc, rules) => acc.concat(...rules), [])
  rules = rules.filter((rule) => el.matches(rule.selectorText))
  rules = rules.map(({ style }) => style)
  return rules
}