Chto eto?

PolyFills.js

/**
 * Array.find
 */
if (!Array.prototype.find) {
    Array.prototype.find = function(predicate) {
        'use strict';
        if (this === null) {
            throw new TypeError('Array.prototype.find called on null or undefined');
        }
        if (typeof predicate !== 'function') {
            throw new TypeError('predicate must be a function');
        }
        let list = Object(this);
        let length = list.length >>> 0;
        let thisArg = arguments[1];
        let value;

        for (let i = 0; i < length; i++) {
            value = list[i];
            if (predicate.call(thisArg, value, i, list)) {
                return value;
            }
        }
        return undefined;
    };
}

/*
 * classList.js: Cross-browser full element.classList implementation.
 * 1.1.20150312
 *
 * By Eli Grey, http://eligrey.com
 * License: Dedicated to the public domain.
 *   See https://github.com/eligrey/classList.js/blob/master/LICENSE.md
 */
/*global self, document, DOMException */
/*! @source http://purl.eligrey.com/github/classList.js/blob/master/classList.js */
if ("document" in self) {
    // Full polyfill for browsers with no classList support
    // Including IE < Edge missing SVGElement.classList
    if (!("classList" in document.createElement("_"))
        || document.createElementNS && !("classList" in document.createElementNS("http://www.w3.org/2000/svg","g"))) {

        (function (view) {

            "use strict";

            if (!('Element' in view)) return;

            let
                classListProp = "classList"
                , protoProp = "prototype"
                , elemCtrProto = view.Element[protoProp]
                , objCtr = Object
                , strTrim = String[protoProp].trim || function () {
                        return this.replace(/^\s+|\s+$/g, "");
                    }
                , arrIndexOf = Array[protoProp].indexOf || function (item) {
                        let
                            i = 0
                            , len = this.length
                        ;
                        for (; i < len; i++) {
                            if (i in this && this[i] === item) {
                                return i;
                            }
                        }
                        return -1;
                    }
                // Vendors: please allow content code to instantiate DOMExceptions
                , DOMEx = function (type, message) {
                    this.name = type;
                    this.code = DOMException[type];
                    this.message = message;
                }
                , checkTokenAndGetIndex = function (classList, token) {
                    if (token === "") {
                        throw new DOMEx(
                            "SYNTAX_ERR"
                            , "An invalid or illegal string was specified"
                        );
                    }
                    if (/\s/.test(token)) {
                        throw new DOMEx(
                            "INVALID_CHARACTER_ERR"
                            , "String contains an invalid character"
                        );
                    }
                    return arrIndexOf.call(classList, token);
                }
                , ClassList = function (elem) {
                    let
                        trimmedClasses = strTrim.call(elem.getAttribute("class") || "")
                        , classes = trimmedClasses ? trimmedClasses.split(/\s+/) : []
                        , i = 0
                        , len = classes.length
                    ;
                    for (; i < len; i++) {
                        this.push(classes[i]);
                    }
                    this._updateClassName = function () {
                        elem.setAttribute("class", this.toString());
                    };
                }
                , classListProto = ClassList[protoProp] = []
                , classListGetter = function () {
                    return new ClassList(this);
                }
            ;
            // Most DOMException implementations don't allow calling DOMException's toString()
            // on non-DOMExceptions. Error's toString() is sufficient here.
            DOMEx[protoProp] = Error[protoProp];
            classListProto.item = function (i) {
                return this[i] || null;
            };
            classListProto.contains = function (token) {
                token += "";
                return checkTokenAndGetIndex(this, token) !== -1;
            };
            classListProto.add = function () {
                let
                    tokens = arguments
                    , i = 0
                    , l = tokens.length
                    , token
                    , updated = false
                ;
                do {
                    token = tokens[i] + "";
                    if (checkTokenAndGetIndex(this, token) === -1) {
                        this.push(token);
                        updated = true;
                    }
                }
                while (++i < l);

                if (updated) {
                    this._updateClassName();
                }
            };
            classListProto.remove = function () {
                let
                    tokens = arguments
                    , i = 0
                    , l = tokens.length
                    , token
                    , updated = false
                    , index
                ;
                do {
                    token = tokens[i] + "";
                    index = checkTokenAndGetIndex(this, token);
                    while (index !== -1) {
                        this.splice(index, 1);
                        updated = true;
                        index = checkTokenAndGetIndex(this, token);
                    }
                }
                while (++i < l);

                if (updated) {
                    this._updateClassName();
                }
            };
            classListProto.toggle = function (token, force) {
                token += "";

                let
                    result = this.contains(token)
                    , method = result ?
                        force !== true && "remove"
                        :
                        force !== false && "add"
                ;

                if (method) {
                    this[method](token);
                }

                if (force === true || force === false) {
                    return force;
                } else {
                    return !result;
                }
            };
            classListProto.toString = function () {
                return this.join(" ");
            };

            if (objCtr.defineProperty) {
                let classListPropDesc = {
                    get: classListGetter
                    , enumerable: true
                    , configurable: true
                };
                try {
                    objCtr.defineProperty(elemCtrProto, classListProp, classListPropDesc);
                } catch (ex) { // IE 8 doesn't support enumerable:true
                    if (ex.number === -0x7FF5EC54) {
                        classListPropDesc.enumerable = false;
                        objCtr.defineProperty(elemCtrProto, classListProp, classListPropDesc);
                    }
                }
            } else if (objCtr[protoProp].__defineGetter__) {
                elemCtrProto.__defineGetter__(classListProp, classListGetter);
            }

        }(self));

    } else {
        // There is full or partial native classList support, so just check if we need
        // to normalize the add/remove and toggle APIs.

        (function () {
            "use strict";

            let testElement = document.createElement("_");

            testElement.classList.add("c1", "c2");

            // Polyfill for IE 10/11 and Firefox <26, where classList.add and
            // classList.remove exist but support only one argument at a time.
            if (!testElement.classList.contains("c2")) {
                let createMethod = function(method) {
                    let original = DOMTokenList.prototype[method];

                    DOMTokenList.prototype[method] = function(token) {
                        let i, len = arguments.length;

                        for (i = 0; i < len; i++) {
                            token = arguments[i];
                            original.call(this, token);
                        }
                    };
                };
                createMethod('add');
                createMethod('remove');
            }

            testElement.classList.toggle("c3", false);

            // Polyfill for IE 10 and Firefox <24, where classList.toggle does not
            // support the second argument.
            if (testElement.classList.contains("c3")) {
                let _toggle = DOMTokenList.prototype.toggle;

                DOMTokenList.prototype.toggle = function(token, force) {
                    if (1 in arguments && !this.contains(token) === !force) {
                        return force;
                    } else {
                        return _toggle.call(this, token);
                    }
                };

            }

            testElement = null;
        }());

    }

}