hasFont.js

// hasFont.js
// Detects whether a font exists on a client with pure JavaScript.
// Released under CC0 1.0 Public Domain Dedication
// https://creativecommons.org/publicdomain/zero/1.0/

// Boolean hasFont(String fontName)
// n.b. fontName should not have double quotation marks, i.e. `"`
// e.g. hasFont('monospace') // true
//      hasFont('Helvetica Nonexistent') // false
var hasFont = (function(doc){

  // Internet Explorer 8: Array.prototype.every polyfill
  // If you don't need to support IE8 or lower, suppress this line.
  Array.prototype.every||(Array.prototype.every=function(r,t){"use strict";var e,n;if(null==this)throw new TypeError;var o=Object(this),i=o.length>>>0;if("function"!=typeof r)throw new TypeError;for(arguments.length>1&&(e=t),n=0;i>n;){var f;if(n in o){f=o[n];var y=r.call(e,f,n,o);if(!y)return!1}n++}return!0});

  var standard = doc.createElement('span');
  var experiment = doc.createElement('span');

  var checksAgainst = ["monospace", "Georgia", "Times New Roman", "Verdana"];
  var alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split('');

  var style = 'opacity: 0; font-size: 99px; position: absolute; top: -19999px; left: -1999px; margin: 0';
  standard.setAttribute('style', style);
  experiment.setAttribute('style', style);

  var identical = function (standard, experiment) {
    return ((standard.offsetHeight === experiment.offsetHeight) &&
            (standard.offsetWidth === experiment.offsetWidth));
  };

  var check = function (fontName) {
    return !checksAgainst.every(function(fallback) {
      standard.style.fontFamily = fallback;
      experiment.style.fontFamily = fontName.trim() + ', ' + fallback;
      return alphabet.every(function(letter){
        standard.innerText = experiment.innerText = Array(70).join(letter);
        return identical(standard, experiment);
      });
    });

  };

  return function (fontName) {
    doc.body.appendChild(standard);
    doc.body.appendChild(experiment);
    var result = check('"' + fontName + '"');
    doc.body.removeChild(standard);
    doc.body.removeChild(experiment);
    return result;
  };

})(document);