https://www.naibaat.pk/assets/frontend/plugins/prr_font_detector/1.0/font_detector.js

1//change font size
2var Detector = function() {
3    // a font will be compared against all the three default fonts.
4    // and if it doesn't match all 3 then that font is not available.
5    var baseFonts = ['monospace', 'sans-serif', 'serif'];
6
7    //we use m or w because these two characters take up the maximum width.
8    // And we use a LLi so that the same matching fonts can get separated
9    var testString = "mmmmmmmmmmlli";
10
11    //we test using 72px font size, we may use any size. I guess larger the better.
12    var testSize = '72px';
13
14    var h = document.getElementsByTagName("body")[0];
15
16    // create a SPAN in the document to get the width of the text we use to test
17    var s = document.createElement("span");
18    s.style.fontSize = testSize;
19    s.innerHTML = testString;
20    var defaultWidth = {};
21    var defaultHeight = {};
22    for (var index in baseFonts) {
23        //get the default width for the three base fonts
24        s.style.fontFamily = baseFonts[index];
25        h.appendChild(s);
26        defaultWidth[baseFonts[index]] = s.offsetWidth; //width for the default font
27        defaultHeight[baseFonts[index]] = s.offsetHeight; //height for the defualt font
28        h.removeChild(s);
29    }
30
31    function detect(font) {
32        var detected = false;
33        for (var index in baseFonts) {
34            s.style.fontFamily = font + ',' + baseFonts[index]; // name of the font along with the base font for fallback.
35            h.appendChild(s);
36            var matched = (s.offsetWidth != defaultWidth[baseFonts[index]] || s.offsetHeight != defaultHeight[baseFonts[index]]);
37            h.removeChild(s);
38            detected = detected || matched;
39        }
40        return detected;
41    }
42
43    this.detect = detect;
44};