Get all font-family on page

font-falimies.js

(function(){
  var fonts = [];
  Array.from(document.styleSheets).forEach(function(styleSheet){
    Array.from(styleSheet.cssRules || []).forEach(function(rule){
      String(rule.style && rule.style.fontFamily).split(/\s*,\s*/).forEach(function(font){
        font = font.replace(/^["'](.*)["']$/, '$1');
        if(font && fonts.indexOf(font) == -1 && !font.match(/undefined|inherit|initial/i)){
          fonts.push(font);
        }
      })
    });
  });
  console.log(fonts.sort());
})();