JSFiddle - React, Tailwind, and code Playground

CSS

@font-face {
    font-family: foo;
    src: url('bar.otf');
}

JavaScript

var fonts = {};

function getFonts (obj) {
    var o = obj || {},
        sheet = document.styleSheets,
        rule = null,
        i = sheet.length, j;
    while( 0 <= --i ){
        rule = sheet[i].rules || sheet[i].cssRules || [];
        j = rule.length;
        while( 0 <= --j ){
            if( rule[j].constructor.name === 'CSSFontFaceRule' ){ // rule[j].slice(0, 10).toLowerCase() === '@font-face'
                o[ rule[j].style.fontFamily ] = rule[j].style.src;
            };
        }
    }
    return o;
}
    
getFonts( fonts );
console.log( fonts );