fittext.js

/*
* FitText.js 1.0 jQuery free version
*
* Copyright 2011, Dave Rupert http://daverupert.com
* Released under the WTFPL license
* http://sam.zoy.org/wtfpl/
* Modified by Slawomir Kolodziej http://slawekk.info
*/
window.addEvent = function (el, type, fn) {
    if (el.addEventListener) {
        el.addEventListener(type, fn, false);
    } else {
        el.attachEvent('on' + type, fn);
    }
};

window.extend = function (obj, ext) {
    for (let key in ext) {
        if (ext.hasOwnProperty(key)) {
            obj[key] = ext[key];
        }
    }

    return obj;
};

window.fitText = function (el, compressor, options) {
    let settings = extend({
        minFontSize: -1 / 0,
        maxFontSize: 1 / 0
    }, options);

    let compress = compressor || 10;
    let maxSize = settings.maxFontSize;
    let minSize = settings.minFontSize;

    function fit(el) {
        function resize() {
            el.style.fontSize = Math.max(Math.min(el.clientWidth / compress, maxSize), minSize) + 'px';
        }

        function tryResize() {
            if (el.clientWidth > 0) {
                resize()
            } else {
                // Wait until client size is properly computed
                setTimeout(tryResize, 10);
            }
        }

        tryResize();

        // Bind events
        // If you have any js library which support Events, replace this part
        // and remove addEvent function (or use original jQuery version)
        addEvent(window, 'resize', resize);
        addEvent(window, 'orientationchange', resize);
    }

    if (el.length) {
        for (let i = 0; i < el.length; i++) {
            fit(el[i]);
        }
    } else {
        fit(el);
    }

    // return set of elements
    return el;
};