Get all heading styles on a page

headingStyles.js

var headings = ['h1','h2','h3', 'h4', 'h5','h6'];

function onlyUnique(value, index, self) { 
  return self.indexOf(value) === index;
}

headings.map(heading => {
  return {
    [heading]: [...document.querySelectorAll(heading)].map(item => window.getComputedStyle(item).fontSize).filter(onlyUnique)
  };
});